Step 1 Sample Test Answers + Explanations 2026
USMLE STEP 1 Free 120 Explanations

Updated May 2026

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Updated as of May 2026

Here are the detailed explanations of the new Free 120 NBME STEP 1 Sample Test Questions so you don’t waste your time trying to find the correct answers and explanations. We had our expert USMLE tutors who scored 260+ on their exams refine these answers and explanations for you so you can spend your valuable time learning!

You can download the Free 120 STEP 1 Sample Test Questions on the USMLE website here. The order of answers and explanations here is based on the online NBME platform (not the pdf) as of May 2026.

Block 1
Block 2
Block 3
Block 4
Block 5
Block 6

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Block 1

Question 1:

The correct answer is A. This 53-year-old woman presents with a 6-year history of hand stiffness and pain, accompanied by joint deformities as shown in the image, which is consistent with rheumatoid arthritis (RA). Testing for anti-citrullinated peptide antibodies (ACPA) is the most specific diagnostic marker for RA.

Rheumatoid arthritis is a chronic autoimmune disease that primarily affects the small joints of the hands and feet. It is characterized by:

Symmetrical joint pain and stiffness: Especially in the morning, lasting more than 30 minutes.
Joint deformities: including ulnar deviation, swan-neck deformities, and boutonnière deformities.
Systemic features: Fatigue and other inflammatory symptoms are common in RA.

Anti-citrullinated peptide antibodies (ACPA), such as anti-cyclic citrullinated peptide (anti-CCP) antibodies, are highly specific for RA and are detected in most patients with the disease. They are associated with more severe disease and are helpful in confirming the diagnosis.

Incorrect answers:
B. This test is used to diagnose primary biliary cholangitis, a liver disease, and is not relevant to joint pain or deformities.

C. HLA-DQ2 is associated with celiac disease, which can present with gastrointestinal symptoms and malabsorption, not joint deformities.

D. Abnormalities in erythroid precursors are associated with hematologic conditions such as anemias, not RA.

E. Abnormalities in thrombopoietic precursors are linked to platelet disorders, which are unrelated to RA.

Question 2:

The correct answer is B. Asymmetric smile. This 80-year-old patient has a pontine infarction, as indicated by the shaded area involving the left ventromedial and ventrolateral caudal pons. The lesion damages the left corticospinal tract and medial lemniscus, leading to contralateral (right-sided) hemiparesis and loss of vibration/two-point discrimination. Simultaneously, it involves the exiting fibers of the abducens nerve (CN VI), preventing left eye abduction, and the facial nerve (CN VII) fascicles, which leads to an ipsilateral peripheral facial palsy presenting as an asymmetric smile. For USMLE, a cross-section of the caudal pons showing a ventromedial/lateral lesion paired with alternating hemiparesis and cranial nerve VI/VII deficits is the classic presentation of a pontine branch stroke of the basilar artery.

Incorrect answers:
A. Anesthesia of the left side of the face would require involvement of the principal sensory nucleus of CN V (mid-pons) or the spinal trigeminal nucleus and tract (lateral pons/medulla). The shaded lesion spares these dorsolateral structures.

C. Hoarseness is caused by a lesion to the nucleus ambiguus or the fibers of CN X (Vagus nerve). These structures are located exclusively in the medulla (e.g., Wallenberg syndrome) rather than the pons.

D. Loss of accommodation is mediated by the Edinger-Westphal nucleus and the oculomotor nerve (CN III). This reflex pathway is localized entirely within the midbrain, which is superior to this pontine lesion.

E. Paralysis of the tongue is caused by a lesion to the hypoglossal nucleus or exiting fibers of CN XII. This nucleus is located medially within the medulla (e.g., medial medullary syndrome) and is not affected by a caudal pontine stroke.

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Question 3:

The correct answer is B. This patient’s findings, including shortness of breath, tachycardia, tachypnea, and deep vein thrombosis (DVT) in the right calf, are consistent with a pulmonary embolism (PE). Risk factors such as long flights and a predisposition to thrombosis (due to genetic factors or medications) are significant. Factor V Leiden mutation is associated with an inability to break down factor V, which leads to thrombosis. Factor V Leiden mutation is the most common cause of hereditary thrombophilia.

Incorrect answers:
A. While antithrombin III deficiency increases the risk of thrombosis, it is usually asymptomatic and far less common than Factor V Leiden. Additionally, higher doses of heparin are needed to produce an effect in patients with antithrombin III deficiency.

C. Hemophilia would result in bleeding, not thrombosis, and is not associated with the clinical presentation described.

D. Protein C deficiency is much less common than Factor V Leiden and would also be associated with a higher risk of thrombosis, but it is not the most likely cause in this case. Usually it occurs in the first few days of warfarin use but this patient has no such history of drug use.

E. Von Willebrand disease is a bleeding disorder and would not cause thrombosis.

Question 4:

The correct answer is C. This patient presents with a 2-week history of fever, throat pain, bilateral tonsillar exudate, and generalized lymphadenopathy, which are consistent with mononucleosis-like symptoms. The negative heterophile antibody test rules out Epstein-Barr virus infection (Choice A). Given the presence of anemia, thrombocytopenia, and leukopenia, HIV is the most likely diagnosis. For USMLE, mononucleosis-like symptoms, suspect EBV, CMV, Toxoplasmosis and HIV.

Incorrect answers:
A. Epstein-Barr virus infection is ruled out by the negative heterophile antibody (Monospot) test.

B. Gonococcal pharyngitis is associated with fever, tonsillar exudate, and localized lymphadenitis, but it does not typically cause generalized lymphadenopathy or the laboratory abnormalities seen in this patient.

D. Lymphogranuloma venereum infection is usually associated with a painless genital ulcer and painful inguinal lymphadenopathy, not the generalized lymphadenopathy or throat symptoms seen here.

E. Streptococcal pharyngitis typically presents with fever, sore throat with exudates, and localized cervical lymphadenopathy. It is an acute infection, unlike this patient’s subacute presentation.

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Question 5:

The correct answer is C. This 60-year-old woman presents with fever, rash, acute kidney injury (AKI), and eosinophilia following 21 days of intravenous oxacillin for Staphylococcus aureus endocarditis. Urine microscopy shows eosinophils and white blood cell (WBC) casts, which are highly suggestive of acute interstitial nephritis (AIN). AIN is a hypersensitivity reaction typically caused by medications, including beta-lactam antibiotics like oxacillin.

Incorrect answers:
A. Collapsing FSGS is associated with conditions like HIV infection, heroin use, or certain medications. It manifests with nephrotic syndrome (severe proteinuria, hypoalbuminemia, and edema) rather than AKI with eosinophiluria and WBC casts.

B. This is characteristic of crescentic glomerulonephritis, a rapidly progressive glomerulonephritis (RPGN) associated with autoimmune diseases like ANCA-associated vasculitis or Goodpasture syndrome. These diseases usually cause nephritic syndrome (hematuria, RBC casts, and hypertension), which is not seen here.

D. This finding is associated with diabetic nephropathy, which develops over years and manifests with proteinuria, not AKI. This patient has no history of diabetes and lacks findings consistent with chronic diabetic nephropathy.

E. This is seen in acute tubular necrosis (ATN), which occurs due to ischemia or nephrotoxins (e.g., aminoglycosides, contrast dye). While ATN may cause AKI, it does not cause eosinophilia, WBC casts, or fever.

Question 6:

The correct answer is D. This patient has symptomatic syringomyelia, as evidenced by the gradual progression of upper extremity weakness and decreased pain sensation. MRI confirms the diagnosis by showing a central syrinx in the cervical spine. A history of trauma increases the risk of syrinx formation. Fine touch and vibratory sense are spared in syringomyelia, except in advanced disease.

Incorrect answers:
A. A strict vegan diet may lead to vitamin B12 deficiency. vitamin B12 deficiency causes subacute combined degeneration which can present as sensory, motor and balance deficits that are distributed all over the body not only limited to the upper extremities.

B. A family history could increase the risk of certain conditions like Rheumatoid arthritis and osteoarthritis.

C. Recent travel increases the risk of deep vein thrombosis (DVT), which typically presents with unilateral leg swelling, pain, and erythema rather than the neurological deficits seen in this patient.

E. Unintended weight loss is commonly associated with malignancies. This patient’s history and MRI findings suggest post-traumatic syringomyelia rather than a neoplastic process.

Question 7:

The correct answer is E. Vascular obstruction by lipid-rich plaques. This 68-year-old patient presents with classic symptoms of peripheral artery disease (PAD), characterized by intermittent claudication (lower extremity pain and cramping induced by walking and relieved by rest). His physical exam demonstrates chronic ischemic changes, including atrophic skin, hair loss, cool extremities, and absent distal pulses (posterior tibial and dorsalis pedis). The fundamental pathophysiological mechanism underlying PAD is atherosclerosis, which involves the progressive narrowing of large and medium-sized arteries due to vascular obstruction by lipid-rich plaques.

Management of symptomatic PAD focuses on cardiovascular risk reduction and symptom relief. Initial treatments include supervised exercise therapy, aggressive risk factor modification with high-intensity statins and blood pressure control, and antiplatelet therapy (e.g., aspirin or clopidogrel) to prevent major adverse cardiovascular events. For patients with persistent claudication despite exercise therapy, a phosphodiesterase inhibitor like cilostazol can be added to improve walking distance.

Incorrect answers:
A. Fibromuscular dysplasia is a non-atherosclerotic, non-inflammatory vascular disease that typically affects young women and primarily involves the renal and internal carotid arteries, leading to secondary hypertension or stroke rather than lower extremity claudication.

B. Giant cell arteritis is a large-vessel vasculitis that typically affects patients older than 50, but classic symptoms involve the branches of the carotid artery, presenting as temporal headaches, jaw claudication, and amaurosis fugax, rather than lower extremity ischemia.

C. Hyaline arteriosclerosis is a disease of small arterioles characteristically seen in long-standing hypertension and diabetes mellitus. While this patient has both risk factors and underlying nephropathy due to arteriolosclerosis, it affects microvasculature rather than the large/medium conduit arteries responsible for lower limb claudication and absent pedal pulses.

D. Segmental inflammation of medium-sized vessels describes Polyarteritis nodosa (PAN) or Thromboangiitis obliterans (Buerger disease). PAN presents with systemic symptoms (fever, weight loss) and renal/gastrointestinal ischemia, while Buerger disease causes digital ischemia and gangrene strictly in young, heavy smokers, which does not align with this patient’s presentation.

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Question 8:

The correct answer is D. Fused commissures. This 31-year-old woman presents with progressive shortness of breath, fatigue, and pulmonary congestion (crackles, dullness to percussion) after emigrating from Nigeria. She has a critical history of a hospital admission for a febrile illness and a heart condition at age 10. This clinical presentation is highly characteristic of chronic rheumatic heart disease affecting the mitral valve, leading to severe mitral stenosis.

Chronic rheumatic carditis develops years after an initial episode of acute rheumatic fever (caused by untreated Streptococcus pyogenes pharyngitis). Recurrent, subclinical immune-mediated inflammation leads to progressive fibrotic thickening, calcification, and fused commissures of the valve leaflets, often accompanied by fusion and shortening of the chordae tendineae. This creates a rigid, narrowed valve orifice (classic fish-mouth deformity) that restricts blood flow from the left atrium to the left ventricle during diastole, causing elevated left atrial pressure that backs up into the pulmonary circulation.

Cardiac Auscultation Findings: On physical examination, this condition is characterized by a unique triad of findings:

Loud first heart sound (S1): Caused by the stiff, fibrotic mitral leaflets snapping shut under increased left atrial pressure at the start of systole.
Opening snap (OS): A high-pitched sound heard early in diastole, caused by the sudden, forceful opening of the stenotic and rigid mitral valve.
Mid-diastolic rumble: A low-pitched rumbling murmur with presystolic accentuation, best heard at the cardiac apex using the bell of the stethoscope with the patient in the left lateral decubitus position.

Incorrect answers:
A & E. Ballooned leaflets and myxomatous degeneration are the pathologic hallmarks of mitral valve prolapse (MVP). This is a benign connective tissue disorder where the leaflets become redundant and floppy, bowing into the left atrium during systole, which produces a mid-systolic click and a late systolic murmur.

B. Bulky cusp vegetations characterize active infective endocarditis. These are friable masses composed of platelets, fibrin, and microorganisms that destroy the valve leaflets, presenting with acute systemic symptoms like high fever, new murmurs, and embolic phenomena.

C. Calcified nodules are characteristic of age-related degenerative (senile) calcific aortic stenosis. This is a wear-and-tear disease that typically presents in patients older than 65 years with a harsh systolic crescendo-decrescendo murmur, not a diastolic issue in a young woman.

Question 9:

The correct answer is A. In motivational interviewing, the physician should acknowledge the patient’s resistance to change and guide them toward motivation for that change. Motivational interviewing must always be patient-centered, so open-ended questions encouraging patients to reflect on the benefits of change are particularly effective; they also help explore the patient’s intrinsic motivations for quitting. Through this approach, the patient can consider both the advantages and risks of making a change, thereby increasing their readiness to act.

Incorrect answers:
B & D. While these statements may be true, motivational interviewing is centered around the patient. These options emphasize the physician’s perspective instead of fostering the patient’s self-motivation.

C & D. These statements blame the patient and may create guilt, which can be counterproductive and harm the patient-physician relationship.

Question 10:

The correct answer is B. Caloric insufficiency. This 18-month-old girl presents with nonorganic failure to thrive due to inadequate nutritional intake. Her length tracks along the 25th percentile, while her weight has fallen below the 5th percentile. Pathophysiologically, when calories are insufficient, the body prioritizes linear bone growth and brain development over adipose tissue accumulation, causing weight to drop first while length is initially preserved.

Management focuses on dietary optimization, such as scheduling structured meals, increasing caloric density, and limiting excessive milk intake (which blunts appetite for solids). Routine lab tests are not indicated unless nutritional counseling fails.

Incorrect answers:
A. Adrenal insufficiency presents with systemic symptoms like vomiting, diarrhea, lethargy, skin hyperpigmentation, or electrolyte abnormalities, which are absent in this healthy child.

C. Constitutional growth delay features a parallel drop in both weight and length percentiles within the first 2 years of life before stabilizing, accompanied by a delayed bone age.

D. Familial short stature presents with both length and weight tracking consistently along a lower percentile from early infancy, matching the short stature of the parents.

E. Growth hormone deficiency primarily impairs linear skeletal growth, causing a progressive decline in the length percentile while weight is relatively preserved.

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Question 11:

The correct answer is A. Internal validity. This prospective cohort study evaluates the relationship between dietary calcium and osteoporosis risk. Adding a missing major calcium source (yogurt) to the questionnaire prevents misclassification bias. Accurately capturing total exposure ensures the study measures what it truly intends to measure, maximizing internal validity.

Internal validity represents the degree to which the observed clinical findings truly reflect the cause-and-effect relationship within the study sample, free from systematic design or measurement flaws.

Incorrect answers:
B. Inter-rater reliability measures the consistency between different observers recording the same data point, which is unaffected by modifying a self-administered questionnaire.

C. Response rate is the percentage of invited individuals who complete the study; adding items does not increase participation and can lower it if the survey becomes too long.

D. Type I error (alpha) is the probability of finding a false positive result, which is managed by adjusting the statistical significance threshold (typically $0.05$) rather than changing questionnaire variables.

Question 12:

The correct answer is D. Reduviid bug. This 40-year-old asymptomatic woman from El Salvador has Chagas disease (Trypanosoma cruzi), which is endemic to Central and South America and classically transmitted by the Reduviid (kissing) bug. The insect feeds at night and defecates near the wound; the host then scratches the infectious feces into the bite site or mucous membranes.

While initially asymptomatic, chronic untreated infection can cause severe long-term complications, including dilated cardiomyopathy, apical aneurysms, megacolon, and megaesophagus.

Incorrect answers:
A. Bedbugs cause localized, pruritic skin wheals but do not transmit systemic pathogens like Trypanosoma cruzi.

B. Black flies transmit Onchocerca volvulus, the tissue nematode responsible for causing river blindness (onchocerciasis).

C. Mosquitos serve as vectors for entirely different diseases, including malaria (Anopheles), dengue, and yellow fever (Aedes)

E. Ticks transmit bacterial and protozoal illnesses such as Lyme disease, Rocky Mountain spotted fever, and babesiosis.

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Question 13:

The correct answer is A. “Help me to understand what you are hoping I can do for you today.” This 40-year-old man with irritable bowel syndrome (IBS) frequently seeks care but continuously refuses conventional treatment, preferring his grandmother’s herbal tea. When handling chronic nonadherence in patients who still actively seek medical encounters, the most appropriate initial strategy is to use open-ended, nonjudgmental questioning to explore their expectations and goals for the visit. This fosters collaboration and uncovers hidden agendas or specific fears.

Ethically, addressing patient autonomy requires exploring the underlying reasons for treatment refusal rather than forcing compliance or prematurely ending the therapeutic relationship.

Incorrect answers:
B. Prescribing medication while demanding the patient reconsider is overly directive, which increases resistance and damages rapport when dealing with chronic nonadherence.

C. Asking why the patient keeps coming back carries a dismissive, adversarial tone that invalidates their distress and can cause complete disengagement from healthcare.

D. Focusing immediately on the herbal tea narrows the discussion too quickly to a specific alternative therapy instead of addressing the broader issue of the patient’s expectations.

E. Stating there is nothing that can be done if medication is refused represents a premature abandonment of the patient, failing to recognize that supportive care, validation, and reassurance provide clinical value.

Question 14:

The correct answer is B. alpha 1 Adrenergic antagonism. This 72-year-old man presents with typical lower urinary tract symptoms (LUTS)—including increased urinary frequency, nocturia, and a slow urinary stream—secondary to benign prostatic hyperplasia (BPH), confirmed by a diffusely enlarged, symmetric prostate on digital rectal examination. First-line medical therapy for rapid symptom relief features alpha 1 adrenergic antagonists (e.g., tamsulosin, doxazosin). These agents block alpha 1 receptors located heavily on the smooth muscle of the prostatic urethra and bladder neck, causing smooth muscle relaxation, decreasing dynamic outflow resistance, and significantly improving urine flow.

Alternative long-term treatment includes 5-alpha reductase inhibitors (e.g., finasteride), which block the conversion of testosterone to dihydrotestosterone (DHT) to reduce overall prostate volume, but require months to become clinically effective.

Incorrect answers:
A. alpha 1 Adrenergic agonism (e.g., phenylephrine, midodrine) induces smooth muscle contraction at the bladder neck, which would worsen urinary obstruction and potentially precipitate acute urinary retention in a patient with BPH.

C. alpha 2 Adrenergic agonism (e.g., clonidine, methyldopa) acts centrally to decrease sympathetic outflow, primarily used as a centrally acting antihypertensive agent rather than a treatment for localized urinary tract symptoms.

D. alpha 2 Adrenergic antagonism historically found use in managing erectile dysfunction by increasing central sympathetic activity, but it plays no role in relaxing prostatic smooth muscle.

E. beta 1 Adrenergic agonism (e.g., dobutamine) exerts positive inotropic and chronotropic effects on the myocardium to manage cardiogenic shock or acute heart failure, carrying no physiological effect on prostatic tissue.

F. beta 1 Adrenergic antagonism (e.g., metoprolol, atenolol) involves beta-blockers utilized for hypertension, ischemic heart disease, and rate control in tachyarrhythmias, without any specific clinical utility in treating BPH.

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Question 15:

The correct answer is A. This patient has manic symptoms with psychotic features as described in this scenario. Manic symptoms include rapid speech, loose associations, and lack of sleep. Psychotic features include hallucinations (hearing voices) and delusions (married to the president’s twin brother, receiving instructions from a higher being). it’s differentiated from Schizoaffective disorder as the latter is characterized by psychotic symptoms at least 2 weeks with the absence of Mood disorder.

Incorrect answers:
B. Brief psychotic disorder involves psychotic symptoms lasting less than 30 days. This patient’s symptoms have persisted for more than 2 months, in addition to manic episodes, ruling out this diagnosis.

C. Delusional disorder is characterized by fixed, false beliefs, but the patient usually functions well otherwise. This patient displays marked impairment and manic features, which exclude this diagnosis.

D. Psychotic disorder due to a general medical condition requires evidence of an underlying medical condition causing the symptoms. No information in this scenario supports such a diagnosis.

E. Schizophrenia requires symptoms (e.g., hallucinations, delusions, disorganized speech) to persist for >6 months. Additionally, the presence of manic symptoms strongly supports a different diagnosis.

Question 16:

The correct answer is B. Cimetidine: B, Omeprazole: A. Both drugs are acid-suppressants that increase gastric pH over time. Omeprazole is a proton pump inhibitor (PPI) that irreversibly binds the H/K ATPase pump, providing profound, long-lasting acid suppression (Curve A). Cimetidine is an H2 receptor antagonist; it blocks histamine-mediated acid secretion but leaves gastrin and acetylcholine pathways partially active, resulting in a more modest, lower peak in pH (Curve B).

Because PPIs completely shut down the final common pathway of acid production, they cause a significantly greater and more sustained rise in gastric pH than H2 blockers.

Incorrect answers:
Curves C & D. These curves show a decrease in pH over time (increased acidity), which represents the physiological response to an acid stimulant (like histamine or gastrin) rather than an acid suppressant.

Options C, D, E, & F. These options incorrectly pair Cimetidine or Omeprazole with acid-inducing curves (C or D), or invert their relative potencies by suggesting Cimetidine elevates pH higher than Omeprazole.

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Question 17:

The correct answer is B. This patient has new metastatic lesions in the liver and lungs. Metastatic spread from the right side of the colon follows this pathway: the superior mesenteric vein drains blood from the right colon into the portal vein, which then enters the liver. From the liver, cancer cells can spread through the hepatic veins, into the inferior vena cava (IVC), and subsequently to the heart (right atrium and right ventricle) and pulmonary vasculature, leading to metastasis in the lungs.

Incorrect answers:
A & C. These veins drain the left side of the colon, NOT the right.

D. The middle colic artery supplies part of the transverse colon and does not contribute to venous drainage or cancer spread.

E. The pulmonary veins carry oxygenated blood from the lungs to the left atrium and are not involved in the metastatic process.

F. Arteries do not play a primary role in the venous spread of metastases.

G. The superior vena cava drains blood from the upper body (head, neck, and upper extremities) and does not play a role in this metastatic process.

Question 18:

The correct answer is E. Scabies. This 4-year-old boy has a classic presentation of scabies, an infestation caused by the mite Sarcoptes scabiei. The hallmark finding is severe pruritus (worse at night) along with short, thin, elevated serpiginous tracks (burrows). The lesions characteristically favor the interdigital web spaces of the hands, axillae, waistline, and inner thighs, with small vesicles marking where the female mite resides.

Incorrect answers:
A. Chickenpox presents with fever and a generalized rash of pruriginous vesicles at different stages of healing (“dewdrops on a rose petal”), not linear tracks.

B. Ehrlichiosis is a tick-borne illness causing systemic symptoms (fever, myalgias) and leukopenia; a rash is rare and never forms burrows.

C. Lyme disease is characterized by erythema migrans, an expanding red rash with central clearing (“bull’s-eye” appearance) at a tick bite site.

D. Pediculosis (lice) involves hair or clothing infestation with intense itching and visible nits (eggs), but does not cause intraepidermal burrows.

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Question 19:

The correct answer is E. This patient with sickle cell disease (SCD) likely has gallstones due to chronic hemolysis, which increases unconjugated bilirubin levels. When bilirubin levels exceed the bile’s solubility capacity, bilirubin stones form a common complication of chronic hemolytic anemias, including SCD. in SCD, patients with functional asplenia are at higher risk for infections against encapsulated organisms. and complications like gallstones.

Incorrect answers:
B. Decreased bile salt reabsorption occurs in malabsorption syndromes (e.g., Crohn’s disease, ileal resection), leading to cholesterol stone formation due to impaired micelle formation, but this is not the mechanism in SCD.

C. Increased cholesterol-to-bile ratio is common in cholesterol stone formation, typically seen in middle-aged, obese, fertile women due to estrogen’s effect on cholesterol metabolism. This does not explain gallstones in an SCD patient.

A & D. These options do not contribute significantly to gallstone formation in this patient’s case.

Question 20:

The correct answer is D. This patient most likely has MEN2A syndrome, which is suggested by the presence of pheochromocytoma and a neck mass likely originating from parafollicular cells, most likely thyroid carcinoma. Additionally, the family history of the disease further supports the diagnosis. MEN2A is associated with mutations in the RET proto-oncogene. MEN2A is associated Thyroid carcinoma, Parathyroid hyperplasia, Pheochromocytoma (TPP)

Incorrect answers:
A. Cell cycle regulation genes include TP53, RB, Cyclins, CDKs. RET oncogene is unrelated.

B. DNA mismatch repair genes include MLH1, MSH2. They are involved in Hereditary non polyposis colorectal cancers. RET oncogene is unrelated

C. Metastasis suppressor genes generally inhibit spread of tumors. RET oncogene in unrelated

E. Tumor suppressor genes include TP53, RB. RET oncogene is unrelated

Block 2

Question 21:

The correct answer is A. This 6-year-old girl presents with vaginal bleeding, breast development, and pubic hair growth, which are signs of precocious puberty. Laboratory findings reveal elevated estradiol and gonadotropins (FSH and LH), particularly a high LH level in response to endogenous stimulation. This pattern suggests activation of the hypothalamic-pituitary-gonadal axis, also known as central precocious puberty. Central activation occurs due to the early maturation of gonadotropin-releasing hormone (GnRH)-secreting neurons in the hypothalamus.

Incorrect answers:
B. Prolactin secretion is unrelated to this patient’s presentation. Ectopic prolactin secretion (e.g., from a pituitary tumor or other sources) leads to galactorrhea and menstrual irregularities in older individuals, not precocious puberty.

C. Exogenous sex steroid exposure could lead to premature development of secondary sexual characteristics. However, in such cases, gonadotropins (FSH and LH) would be low due to suppression of the hypothalamic-pituitary axis.

D. A germline mutation in GNAS causes McCune-Albright syndrome, which is characterized by peripheral precocious puberty, café-au-lait spots, and fibrous dysplasia of the bones. In McCune-Albright syndrome, gonadotropins are typically suppressed (low FSH and LH), as estradiol is elevated independently of the hypothalamic-pituitary axis. This patient has no evidence of café-au-lait spots or skeletal abnormalities, and her gonadotropin levels are elevated, making this diagnosis unlikely.

E. 21-hydroxylase deficiency, the most common form of congenital adrenal hyperplasia, leads to excess adrenal androgens and ambiguous genitalia in females at birth or early virilization in childhood. This condition causes low cortisol, high ACTH, and elevated androgens (testosterone, androstenedione). This patient has normal genitalia, a normal ACTH level, and no evidence of hyperandrogenism.

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Question 22:

The correct answer is E. This 16-year-old male with delayed puberty, absence of pubic and facial hair, small penis and testicular size, and tall stature with a high BMI is most likely presenting with Klinefelter syndrome (47,XXY), which is caused by nondisjunction of the X chromosome during meiosis.

Klinefelter syndrome is a chromosomal disorder characterized by an extra X chromosome (47,XXY). It is one of the most common causes of primary hypogonadism in males. This patient’s clinical features, including tall stature, eunuchoid body habitus, small testicles, and lack of secondary sexual characteristics (e.g., facial and pubic hair), are classic findings. The underlying pathology is testicular dysfunction, leading to low testosterone levels and elevated gonadotropins (LH and FSH) due to a lack of negative feedback.

Incorrect answers:
A. This condition involves delayed onset of puberty with normal eventual development. However, this patient has physical findings (e.g., small testicles) indicative of primary hypogonadism, not just a constitutional delay.

B. This refers to conditions like Kallmann syndrome, which causes secondary hypogonadism. However, patients with Kallmann syndrome typically have a normal-sized testicle and anosmia, which are absent here.

C. This would result in primary hypogonadism but is less common and would not explain the constellation of tall stature and eunuchoid features.

D. This mutation causes congenital adrenal hyperplasia (CAH), which is associated with androgen excess, not deficiency. Male patients with CAH typically present with early virilization, not delayed puberty.

Question 23:

The correct answer is B. This newborn with ambiguous genitalia, hyponatremia, hyperkalemia, hypoglycemia, and metabolic acidosis is likely experiencing congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency (most common form of CAH). This deficiency leads to impaired synthesis of cortisol and aldosterone, resulting in elevated ACTH, adrenal hyperplasia, and increased androgen production. The most appropriate pharmacotherapy is mineralocorticoid (aldosterone) and glucocorticoid replacement to address the hormonal imbalances.

Incorrect answers:
A. ACTH is elevated in this patient due to a lack of feedback inhibition by cortisol. Administering ACTH would worsen adrenal hyperplasia and increase androgen overproduction. The treatment goal is to suppress ACTH secretion by replacing deficient cortisol and aldosterone.

C. Androgen therapy is not appropriate because this patient already has excessive androgen production due to 21-hydroxylase deficiency. The goal of treatment is to reduce androgen overproduction by providing glucocorticoids to suppress ACTH.

D. The patient’s symptoms stem from defective adrenal steroidogenesis, not gonadotropin abnormalities. GnRH regulates the hypothalamic-pituitary-gonadal axis and is unrelated to the adrenal insufficiency seen in CAH.

E. Growth hormone deficiency does not explain this patient’s presentation. The ambiguous genitalia, electrolyte abnormalities, and hypoglycemia are classic for adrenal insufficiency due to 21-hydroxylase deficiency, not growth hormone deficiency.

Question 24:

The correct answer is C. Pericardial friction rub. This 59-year-old man has advanced chronic kidney disease (CKD) with severe uremia (BUN 65 mg/dL). A pericardial friction rub indicates uremic pericarditis, which is a life-threatening complication of renal failure and a definitive, absolute indication for immediate hemodialysis.

Incorrect answers:
A. Increasingly severe anemia is a common chronic complication of CKD due to decreased erythropoietin production; it is managed with recombinant erythropoietin and iron supplements, not emergent dialysis.

B. Moderate pedal edema reflects mild fluid overload in CKD, which is initially managed conservatively with loop diuretics rather than acute hemodialysis.

D. Persistent hypertension is a chronic manifestation of progressive CKD managed by optimizing multi-drug antihypertensive regimens (such as ACE inhibitors or ARBs) and fluid restrictions.

E. Recurrent episodes of hypoglycemia can occur in advanced CKD due to reduced renal clearance of insulin, requiring down-titration of his diabetes medications rather than dialytic intervention.

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Question 25:

The correct answer is C. The points on the same curve represent the same contractility, meaning they correspond to the appropriate end-diastolic volume for a given cardiac output. The slope of the curve represents contractility, so any transition along the same curve (e.g., W to Y) occurs with unchanged contractility. This patient presents with findings suggestive of an acute STEMI in the anterolateral leads (ST-segment elevation in V4 to V6, new Q waves, and increased serum troponin I). In anterolateral STEMI, the left ventricular function is compromised due to reduced blood supply, leading to decreased contractility. The resulting decreased contractility reduces stroke volume, which decreases cardiac output and increases end-diastolic volume (blood left in the ventricle due to impaired ejection).

Question 26:

The correct answer is E. Spleen. The abdominal CT scan reveals a bright, metallic bullet causing a streak artifact on the left side of the upper abdomen. Anatomically, this solid organ located lateral to the stomach and adjacent to the left 10th intercostal space is the spleen.

Incorrect answers:
A. The left adrenal gland is a small, retroperitoneal structure located medially and superiorly to the kidney, not laterally.

B. The colon sits more anteriorly or inferiorly at this slice level and typically contains visible pocketed gas or stool.

C. The left kidney is situated more medially and posteriorly; the image shows the bullet passed lateral to it.

D. The lung bases are visible posteriorly at this thoracic-abdominal junction, but the bullet is lodged entirely within the solid abdominal organ.

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Question 27:

The correct answer is C. This 66-year-old man with Parkinson disease is on carbidopa-levodopa therapy but continues to experience motor fluctuations such as freezing episodes, rigidity, and clumsiness. The adjunct therapy in question is most likely a MAO-B inhibitor, which helps enhance dopaminergic signaling by reducing the breakdown of dopamine in the brain.

Parkinson’s disease treatment typically begins with levodopa, a precursor of dopamine, in combination with carbidopa, which inhibits peripheral conversion of levodopa to dopamine, allowing more levodopa to reach the brain. MAO-B inhibitors (e.g., selegiline, rasagiline) are often used as adjunct therapy in Parkinson’s disease. These drugs inhibit the breakdown of dopamine in the brain, prolonging its effects and improving motor symptoms. This patient has on off phenomenon due to fluctuations in medication effectiveness, which cause patients to transition between periods of improved mobility (i.e., “on”) and the return of symptoms such as rigidity and tremors (i.e., “off”). adding monoamine oxidase type B inhibitor (e.g., selegiline or rasagiline), which prevents the central conversion of dopamine into DOPAC (i.e., 3,4-dihydroxyphenylacetic acid), can prolong the effect of the neurotransmitter.

Incorrect answers:
A. This enzyme converts levodopa to dopamine. Carbidopa already inhibits this enzyme peripherally, so further inhibition is unnecessary.

B. This enzyme converts dopamine to norepinephrine. It is unrelated to Parkinson’s disease treatment.

D. This enzyme converts norepinephrine to epinephrine and is unrelated to dopaminergic signaling in Parkinson disease.

E. This is the rate-limiting enzyme in dopamine synthesis. Inhibition of this enzyme would worsen Parkinson’s symptoms by decreasing dopamine synthesis.

Question 28:

The correct answer is B. Muscle rigidity. This 33-year-old patient has developed Neuroleptic Malignant Syndrome (NMS), a life-threatening reaction to antipsychotic medication (olanzapine). NMS is characterized by altered mental status, autonomic instability (tachycardia, extreme hypertension, tachypnea), hyperthermia, and generalized “lead-pipe” muscle rigidity.

Incorrect answers:
A. Hyperreflexia and neuromuscular clonus are highly characteristic of Serotonin Syndrome rather than NMS. NMS typically presents with sluggish or decreased deep tendon reflexes due to severe muscle rigidity.

C. Mydriasis (dilated pupils) is typical of Serotonin Syndrome or sympathomimetic toxicity. In contrast, the pupillary examination in a patient with NMS is characteristically normal.

D. Petechial rash points toward systemic infectious etiologies such as meningococcemia or infective endocarditis. It is not a feature of idiosyncratic drug reactions like NMS.

E. Unilateral hemiparesis is a focal neurological deficit indicating an acute structural brain lesion like a stroke. NMS presents with symmetric, generalized neuromuscular changes instead.

Question 29:

The correct answer is E. This patient most likely has transitional cell carcinoma (TCC) of the renal pelvis, also known as urothelial carcinoma. Risk factors for this condition include smoking and occupational exposure to naphthylamine. The presentation fits with the typical features of TCC, including hematuria and possible associated symptoms. For USMLE, removed organs most likely indicate a malignant tumor or non-viable tissues. For example, in case of oncocytoma, the kidney is unlikely to be removed.

Incorrect answers:
A. Angiomyolipoma is associated with tuberous sclerosis, but there are no other features of this syndrome present in this case.

B. Melanoma would typically present with a pigmented lesion, often on the skin, and its risk factors include sun exposure and fair skin. This patient’s symptoms do not fit the presentation of melanoma.

C. Nephroblastoma (Wilms tumor) is most commonly seen in children, especially between the ages of 2 and 4. It is unlikely to present in an adult patient.

D. Oncocytomas are benign renal masses usually discovered incidentally during imaging for other conditions. They do not present with symptoms like those seen in this case.

Question 30:

The correct answer is A. Effect of alcohol on the condition. The clinical image and vignette demonstrate papulopustular rosacea, characterized by persistent centrofacial erythema, flushing, and a warm or burning sensation. Common triggers for rosacea flares include emotional stress, temperature changes, spicy foods, and alcohol (especially red wine, which this patient drinks weekly). Asking about the specific effect of alcohol helps confirm the clinical diagnosis by establishing a known lifestyle trigger.

Incorrect answers:
B. Excessive sweating is associated with conditions like hyperhidrosis or generalized autonomic dysfunction, but it is not a primary diagnostic criterion or a well-established precipitating factor for rosacea flares.

C. Family history of adenomas is relevant for genetic tumor syndromes like Lynch syndrome or FAP, which have no pathophysiological link or clinical association with rosacea or its centrofacial erythema.

D. Numbness of the fingers and toes evaluates for peripheral neuropathy, a common long-term complication of her type 1 diabetes mellitus. However, this is unrelated to the workup or confirmation of her acute dermatological condition.

E. Poor healing of skin lacerations checks for microvascular complications and poor glycemic control in patients with diabetes, but it does not aid in diagnosing or identifying the triggers of rosacea.

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Question 31:

The correct answer is B. Herd immunity. This 5-year-old unvaccinated girl has maintained good health because a high percentage of the surrounding population is vaccinated. Herd immunity occurs when a critical threshold of a community is immune to an infectious disease, indirectly protecting susceptible, unvaccinated individuals by reducing the overall chain of transmission.

Incorrect answers:
A. Community eradication of vaccine-preventable disease does not explain her ongoing safety, as very few diseases (such as smallpox) have been globally eradicated; most pathogens continue to circulate globally and can be reintroduced.

C. Lingering protection of maternal antibodies is incorrect because maternal IgG antibodies transferred across the placenta typically degrade and disappear within the first 6 to 12 months of life, offering no protection by age 5.

D. A normal immune system protects against opportunistic infections but cannot inherently prevent primary infection when an unvaccinated individual is exposed to highly contagious pathogens like measles or varicella.

E. Underdiagnosis of vaccine-preventable conditions is incorrect because public health surveillance for severe childhood vaccine-preventable diseases is robust, and her history explicitly notes she has never actually had a vaccine-preventable illness.

Question 32:

The correct answer is A. Binding to the D antigen on fetal erythrocytes. This Rh-negative mother is already alloimmunized, as indicated by a positive postpartum indirect Coombs titer of 1:32 from her first pregnancy. Rho(D) immune globulin (RhIg) contains anti-D IgG antibodies. When administered, these exogenous antibodies cross the placenta and target, coat, and cause the destruction of Rh-positive fetal RBCs in maternal circulation before the mother’s immune system can recognize them, specifically by binding to the D antigen on fetal erythrocytes.

Incorrect answers:
B. Blockade of complement binding to fetal erythrocytes is incorrect because RhIg functions via opsonization and subsequent macrophage clearance in the maternal spleen, rather than by altering complement activation pathways.

C. Induction of class switching by maternal B lymphocytes is incorrect because RhIg aims to prevent maternal B-cell activation entirely; it does not promote or accelerate immunoglobulin class switching.

D. Inhibition of production of fetal blood group A erythrocytes is incorrect because RhIg target specificity is limited entirely to the Rh(D) surface antigen and has no physiological impact on ABO erythrocyte production in the fetal bone marrow.

E. Prevention of fetal erythrocytes from entering maternal blood circulation is incorrect because RhIg cannot physically block or prevent fetomaternal hemorrhage, which naturally occurs during deliveries, trauma, or invasive procedures.

Question 33:

The correct answer is D. Purine degradation pathway. This 40-year-old man presents with acute podagra (edema, erythema, and exquisite tenderness of the right great toe) following alcohol and seafood consumption, which is a classic presentation of acute gouty arthritis. Gout is caused by the precipitation of monosodium urate crystals in joints due to hyperuricemia. Uric acid is the final product of the purine degradation pathway, and its accumulation is driven by dietary purines (seafood like lobster) and decreased renal excretion accelerated by alcohol consumption.

Incorrect answers:
A. Chylomicron processing pathway is involved in the transport of dietary lipids from the intestine to peripheral tissues. Flaws in this pathway lead to hyperchylomicronemia and eruptive xanthomas or pancreatitis, not urate crystal deposition.

B. Methionine regeneration pathway converts homocysteine back into methionine. Disruption of this pathway results in homocystinuria, which presents with ectopia lentis, marfanoid habitus, and early thromboembolism.

C. Pentose phosphate pathway generates NADPH and ribose-5-phosphate. While its overactivity could theoretically increase ribose-5-phosphate availability for purine synthesis, the direct cause of gout is an accumulation of downstream degradation pathway intermediates (uric acid).

E. Urea cycle functions to convert toxic ammonia into non-toxic urea for renal excretion. Inborn errors or failure of this cycle manifest as hyperammonemia, causing lethargy, vomiting, and progressive encephalopathy rather than localized arthritis.

Question 34:

The correct answer is A. “Do you have any concerns about your alcohol use?”. When a patient discloses a substance-related adverse event (such as a DUI), the physician must explore potential problematic drinking using open-ended, nonjudgmental inquiry. Asking “Do you have any concerns about your alcohol use?” is an effective first step in motivational interviewing. It builds therapeutic rapport, prevents defensiveness, and allows the clinician to gauge the patient’s insight and current stage of change (e.g., precontemplation vs. contemplation).

Incorrect answers:
B. “How has your alcohol use been this week compared to last week…” is a closed-ended, narrow comparative question. This style limits the patient’s ability to elaborate and is best reserved for gathering specific quantitative data after initial rapport and insight have been established.

C. “So you’re here because you’re concerned about your alcohol use…” is an assumption-based, presumptive statement. Misrepresenting the patient’s stated reason for the visit (which was a routine exam) can provoke defensiveness and damage the physician-patient dynamic.

D. “When was your last drink?” is a direct, closed-ended question used to map the timeline of recent consumption. This is clinically vital when managing a patient at acute risk for alcohol withdrawal syndrome (characterized by tremors, tachycardia, or diaphoresis), which is absent on this patient’s normal physical exam.

E. “Would you be willing to go through a detoxification program?” is a premature, action-oriented intervention. Supervised detoxification is indicated for patients with severe physiological dependence. Recommending intensive treatment before assessing the patient’s readiness to change typically breeds resistance.

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Question 35:

The correct answer is C. Mesoderm. This 4-year-old girl has primary lymphedema, as evidenced by slowly progressive swelling of her lower extremity and lymphoscintigraphy showing anomalous development of the lymphatic vessels. The cardiovascular system, including the vascular endothelium, lymphatic vessels, microcirculation, and spleen, is entirely derived from the embryonic mesoderm.

Incorrect answers:
A. Ectoderm gives rise to the central and peripheral nervous systems, the epidermis of the skin, hair, nails, subcutaneous glands, and the lens of the eye, rather than the deep endothelial or lymphatic networks.

B. Endoderm differentiates into the epithelial linings of the gastrointestinal tract, respiratory tract, urinary bladder, and urethra, as well as parenchyma of organs like the liver and pancreas.

D. Neural crest cells (a specialized population derived from ectoderm) migrate to form structures such as the peripheral ganglia, melanocytes, craniofacial cartilage, and adrenal medulla, but do not differentiate into peripheral lymphatics.

E. Neuroectoderm develops directly into the central nervous system (brain and spinal cord), neurohypophysis, pineal gland, and the retinas, rather than peripheral mesenchymal systems.

Question 36:

The correct answer is B. This patient has obstructive sleep apnea (OSA) as evidenced by daytime somnolence. The patient also has features of obesity hypoventilation syndrome (OHS), including severe obesity (BMI of 63), cyanosis, and abnormal arterial blood gas findings indicating respiratory acidosis (pH 7.31, high PCO2 of 70, and low PO2). Chronic hypoxia stimulates hypoxia-inducible factor 2 (HIF-2), which increases erythropoietin production and results in elevated hemoglobin and red blood cell mass. OSA patients may present with Hypertension (due to hypoxia), Depression or Atrial fibrillation (due to HTN, Atrial enlargement).

Incorrect answers:
A. In respiratory acidosis, the proper metabolic compensation is metabolic alkalosis, not acidosis. This patient’s bicarbonate would be elevated, not decreased.

C. Obesity causes extrinsic restrictive lung disease, which reduces total lung capacity (TLC). Therefore, TLC should be decreased, not increased.

D. Chronic hypoxia can lead to generalized pulmonary vasoconstriction, causing pulmonary hypertension, which increases pressure in the right ventricle and leads to right ventricular hypertrophy. The left ventricle is not typically affected in this scenario.

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Question 37:

The correct answer is B. This 5-year-old girl presents with bloody diarrhea, anemia, thrombocytopenia, acute kidney injury (elevated creatinine), and hypertension. These findings are consistent with hemolytic uremic syndrome (HUS), which is most commonly caused by Shiga toxin-producing Escherichia coli (STEC), particularly the O157:H7 serotype.

HUS is a triad of:

Microangiopathic hemolytic anemia: Evidenced by pallor, low hemoglobin (8.5 g/dL), and schistocytes (fragmented RBCs) on a peripheral blood smear.
Thrombocytopenia: Low platelet count (45,000/mm³) due to platelet consumption in small thrombi.
Acute kidney injury (AKI): Elevated creatinine (3.3 mg/dL), oliguria, and hypertension due to renal vascular damage.

HUS is typically triggered by infection with Shiga toxin-producing E. coli (STEC), which releases Shiga-like toxin. The toxin:

Damages endothelial cells in the glomeruli, leading to platelet aggregation and microthrombi formation.
Causes hemolysis as red blood cells are sheared passing through damaged vessels.
Results in renal failure due to microvascular occlusion in the kidneys.

The clinical course:

Initial episode of bloody diarrhea (due to intestinal damage caused by the STEC).
Progression to HUS within a week, as seen in this patient with worsening kidney function and anemia after the diarrhea subsided.

Incorrect answers:
A. Although Campylobacter jejuni can cause bloody diarrhea, it is not associated with HUS.

C. Rotavirus typically causes non-bloody diarrhea in young children and is not associated with HUS.

D. Salmonella can cause gastroenteritis and bacteremia but does not produce Shiga toxin or cause HUS.

E. Yersinia pestis causes plague, a systemic infection associated with fever, lymphadenopathy, and sepsis, not diarrhea or HUS.

Question 38:

The correct answer is C. This patient’s intermittent headache is likely a symptom of poorly controlled hypertension (154/100 mmHg). The ECG findings of tall R waves in leads V5-V6 and left axis deviation are indicative of left ventricular hypertrophy (LVH). LVH develops as an adaptive response to chronic hypertension, where there is increased synthesis of contractile filaments (actin and myosin) within existing myocytes, leading to myocyte hypertrophy rather than hyperplasia. In chronic HTN, sarcomeres are added in parallel.

Incorrect answers:
A. Excessive accumulation of glycogen is characteristic of glycogen storage diseases such as Pompe disease, which typically causes cardiomyopathy or systemic symptoms unrelated to the hypertrophic changes seen in this patient.

B. Fibrosis of intraventricular conduction pathways can lead to conduction abnormalities and arrhythmias. However, the occasional skipped beats described in this patient are more consistent with benign premature atrial contractions (PACs) and do not explain the LVH.

D. Misfolding and aggregation of cytoskeletal proteins result in dilated cardiomyopathy (DCM), not LVH. DCM is characterized by ventricular dilation and systolic dysfunction, which are not evident in this patient.

E. Myocyte hyperplasia refers to an increase in the number of myocytes, which is not a mechanism for hypertrophy. In LVH, the adaptive response involves increased actin and myosin synthesis within existing myocytes rather than cell proliferation.

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Question 39:

The correct answer is B. Cytochrome P450. This scenario describes the metabolic activation of pro-carcinogens. Benzene and polycyclic aromatic hydrocarbons (found in cigarette smoke) are not inherently carcinogenic; they must first be oxidized and biotransformed by Phase I microsomal enzymes—specifically the Cytochrome P450 system (such as CYP2E1 for benzene)—into highly reactive, electrophilic intermediates (like benzene oxide) that bind to DNA and cause mutations.

Incorrect answers:
A. Aldehyde dehydrogenase converts toxic aldehydes into non-toxic carboxylic acids (e.g., acetaldehyde to acetate during ethanol metabolism). It is involved in detoxification rather than the harmful metabolic activation of polycyclic hydrocarbons or benzene.

C. Glutathione S-transferase is a Phase II conjugation enzyme that couples reduced glutathione to electrophilic compounds. It acts to neutralize and detoxify reactive intermediates, so a polymorphism reducing its activity—not one causing activation—would increase cancer risk.

D. Methyltransferase is a Phase II enzyme responsible for transferring methyl groups to substrate molecules (such as TPMT or COMT). It is primarily involved in inactivating drugs or neurotransmitters rather than activating environmental pro-carcinogens.

E. Succinate dehydrogenase is an inner mitochondrial membrane enzyme that participates simultaneously in the citric acid cycle and the electron transport chain (Complex II). It plays no role in the metabolic activation or clearance of xenobiotics like benzene.

Question 40:

The correct answer is B. Phase 1.This investigator is assessing the maximum tolerated dose, safety profile, and pharmacokinetics of a new antimalarial drug in a small cohort (80 healthy volunteers). This perfectly describes Phase 1 of clinical drug trials, which focuses on assessing safety, toxicity, pharmacokinetics, and pharmacodynamics in a small group of healthy individuals (or patients with advanced disease for oncology trials) before moving into larger patient populations.

Incorrect answers:
A. Phase 0 consists of microdosing studies conducted very early in human development using subtherapeutic doses in a tiny cohort (<15 people) to evaluate basic pharmacokinetics and cellular targets, not to determine the maximum tolerated dose.

C. Phase 2 clinical trials expand testing to a moderate group of patients (typically 100–300) who actually have the target disease (e.g., individuals with active malaria infection) to evaluate optimal dosing strategies and initial treatment efficacy.

D. Phase 3 trials involve large-scale, often multicenter randomized controlled testing in hundreds to thousands of affected patients. The primary goal is to definitively confirm efficacy, compare the new drug against the current standard of care, and detect rare adverse profiles.

E. Phase 4 clinical trials comprise post-marketing surveillance conducted after the drug has received regulatory FDA approval. This phase monitors the long-term safety, real-world effectiveness, and very rare side effects of the drug across the general population over time.

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Block 3

Question 41:

The correct answer is A. Depletion of glycogen stores. This 21-year-old patient with type 1 diabetes mellitus presents with severe, symptomatic hypoglycemia (serum glucose 25mg/dL, lethargy, cool and clammy skin) after training for a marathon. Glucagon rescue works by binding to hepatic G-protein coupled receptors, activating adenylyl cyclase, increasing cAMP, and rapidly triggering hepatic glycogenolysis to release glucose into the bloodstream. However, prolonged and intense physical exertion (like marathon training) completely exhausts hepatic glycogen reserves, leading to depletion of glycogen stores and rendering exogenous glucagon ineffective in raising blood glucose levels.

Incorrect answers:
B. Downregulation of glucagon receptors by exercise does not occur; instead, prolonged exercise and physiological stress increase sensitivity to counterregulatory hormones like glucagon and epinephrine to optimize fuel mobilization.

C. Impairment of glycogenolysis caused by an increased circulating concentration of catecholamines is biochemically incorrect. Circulating catecholamines (epinephrine and norepinephrine) actively stimulate and accelerate hepatic glycogenolysis via beta2 adrenergic receptors.

D. Inhibition of gluconeogenesis caused by an increased circulating insulin concentration is an effect of insulin, but it does not explain why the acute administration of a glucagon injection failed to break down glycogen and raise blood sugar within 20 minutes in an exhausted patient.

Question 42:

The correct answer is B. “I’m concerned the mass could be cancer. The biopsy will provide more information.”. This 72-year-old woman has clear signs of advanced pancreatic cancer (painless jaundice, weight loss, a mass in the head of the pancreas, and liver lesions). When delivering difficult medical news or discussing highly suspicious findings, the physician must communicate with honesty, clarity, and compassion. Using a direct but empathetic “warning shot” statement like “I’m concerned the mass could be cancer” avoids medical jargon, honors the ethical principle of patient autonomy, and accurately prepares the patient for the likely diagnosis without causing unnecessary delay or false reassurance.

Incorrect answers:
A. “I suspect you have a malignant mass… but I’ll need the biopsy results to confirm my suspicion.” uses blunt, overly cold clinical phrasing (“malignant mass”) instead of conversational, patient-centered language, which can impair therapeutic rapport during a highly stressful discussion.

C. “This could be something serious, but let’s not worry about that until we know for sure.” utilizes a paternalistic, dismissive tone that minimizes the clinical reality. Telling a patient “not to worry” invalidates her prospective emotions and ignores the high likelihood of malignancy already visible on the CT scan.

D. “This mass could be benign or malignant. I’d like to wait for the biopsy results before I share the suspected diagnosis.” is evasive and dishonest given the presentation. The presence of a pancreatic head mass alongside liver lesions makes a benign etiology highly improbable, and withholding the suspected diagnosis violates the principle of informed consent.

E. “We don’t know what this mass is yet, but the biopsy should give us some answers.” is overly vague and understates the clinical picture. Failing to mention the strong possibility of cancer leaves the patient completely blindsided and unprepared when the definitive biopsy results return.

Question 43:

The correct answer is B. Janus kinase 2. This 56-year-old man presents with headaches, fatigue, splenomegaly (spleen palpated 2 cm below the costal margin), and panmyelosis—elevations across multiple myeloid cell lines, including significant erythrocytosis (hemoglobin 19.5 g/dL, hematocrit 59%) and thrombocytosis (platelets 450,000 mm). This clinical picture is highly diagnostic of Polycythemia Vera (PV), a chronic myeloproliferative neoplasm. Over 95% of PV cases are driven by a somatic gain-of-function mutation in the Janus kinase 2 (JAK2) gene, which renders hematopoietic growth factor receptors constitutively active independent of erythropoietin levels.

Incorrect answers:
A. G proteins participate in transmembrane signaling for various G-protein coupled receptors (GPCRs). Activating mutations in G proteins (like Gs alpha) are linked to conditions such as McCune-Albright syndrome, not chronic myeloproliferative disorders like polycythemia vera.

C. Platelet-derived growth factor (PDGF) receptor alterations or mutations are characteristically associated with other specific hematologic malignancies, such as chronic myelomonocytic leukemia (CMML) or hypereosinophilic syndrome, rather than classic polycythemia vera.

D. Retinoic acid receptor alpha (RAR alpha) gene translocation t(15;17) fuses it with the PML gene to cause Acute Promyelocytic Leukemia (APL). APL presents acutely with severe coagulopathy (DIC) and blast cells on peripheral smear rather than chronic panmyelosis.

E. Thrombopoietin receptor (MPL) mutations can be found in a minority of patients with essential thrombocythemia or primary myelofibrosis, but they are not the driving mutation in polycythemia vera, which is overwhelmingly caused by JAK2 mutations.

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Question 44:

The correct answer is E. This patient has temporal lobe epilepsy, characterized by focal impaired awareness seizures associated with olfactory hallucinations (“burning rubber”), automatisms (lip smacking), and emotional disturbances. These findings localize the seizure focus to the mesial temporal lobe, particularly the hippocampus and amygdala. In patients with medically refractory temporal lobe epilepsy, intracranial EEG electrodes are commonly placed in the hippocampal region to identify a surgically resectable seizure focus. On this coronal section, label E corresponds to the hippocampus.

Incorrect answers:
A. This region corresponds to the medial frontal/parietal cortex adjacent to the longitudinal fissure. Seizures originating from this area typically produce contralateral leg motor symptoms, such as tonic posturing or jerking, rather than olfactory auras and automatisms.

B. This structure represents the corpus callosum/septal region, a commissural white matter tract connecting the cerebral hemispheres. It is not a common epileptogenic focus in temporal lobe epilepsy.

C. This midline structure corresponds to the thalamic/hypothalamic region. Although the thalamus may participate in seizure propagation, it is not the classic origin of focal seizures associated with olfactory hallucinations and lip smacking.

D. This area represents the lateral temporal cortex/insular region. Although temporal neocortical seizures can arise here, olfactory auras and automatisms are more characteristic of mesial temporal lobe involvement.

Question 45:

The correct answer is A. This young woman exhibits signs of an eating disorder, expressing feelings of worthlessness and believing she is overweight. Additionally, she displays physical symptoms such as swelling of the parotid and submandibular glands, tooth discoloration, and scarring on the dorsum of her right hand, known as Russell’s sign. These findings align with bulimia nervosa.

Bulimia nervosa is an eating disorder that primarily affects young females. It is characterized by recurrent episodes of binge eating, followed by feelings of guilt and compensatory behaviors such as self-induced vomiting or laxative use. Frequent vomiting can lead to excessive saliva production, which causes enlargement of the parotid and submandibular glands, as seen in this patient. Anticholinergic drugs like atropine can reduce salivation, potentially helping to relieve gland enlargement.

Incorrect answers:
B. Drugs that bind to nicotinic acetylcholine receptors are utilized for smoking cessation; however, this patient’s symptoms are unrelated to smoking.

C, D & E. These drugs work through mechanisms of different antibiotics, but this patient’s symptoms are not caused by a bacterial infection.

Question 46:

The correct answer is D. Type IV (delayed). This 42-year-old man presents with a severely pruritic, erythematous, vesicular rash that developed 1 day after cutting weeds in his backyard. The clinical photograph and history describe classic allergic contact dermatitis (poison ivy or related weed exposure causing urushiol-induced dermatitis). This condition is mediated by a Type IV (delayed-type) hypersensitivity reaction, where T-lymphocytes (CD4+ helper T cells and CD8+ cytotoxic T cells) are sensitized to an antigen and subsequently release cytokines upon re-exposure, recruiting macrophages and causing localized epidermal damage.

Incorrect answers:
A. Type I (immediate) hypersensitivity is mediated by IgE antibodies binding to mast cells and basophils, triggering immediate histamine release. It underpins anaphylaxis, urticaria, and acute asthma, presenting within minutes to hours rather than a delayed 24–48 hour timeline.

B. Type II (complement-mediated cytotoxic) hypersensitivity involves IgG or IgM antibodies binding to fixed cell-surface antigens, activating the complement cascade or causing antibody-dependent cellular cytotoxicity (e.g., autoimmune hemolytic anemia, Goodpasture syndrome).

C. Type III (immune complex-mediated) hypersensitivity is driven by the deposition of soluble antigen-antibody (IgG/IgM) complexes in vessel walls, activating complement and attracting neutrophils (e.g., serum sickness, systemic lupus erythematosus, Arthus reaction).

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Question 47:

The correct answer is E. This 45-year-old woman, who lived at a high altitude (10,000 feet above sea level) for two years, presents with signs of right-sided heart failure, including jugular venous distention and pedal edema. Her history and physical findings suggest a prior adaptation to chronic hypoxemia at high altitude, which led to right ventricular hypertrophy (RVH) due to chronic pulmonary hypertension. Over time, the associated hypertrophy likely led to a decrease in right ventricular diastolic compliance, which is the most likely cause of her symptoms.

At high altitudes, oxygen levels in the air are lower, leading to chronic hypoxemia. The physiological response to chronic hypoxemia includes:

Hypoxic pulmonary vasoconstriction: This increases pulmonary vascular resistance and pulmonary artery pressure.
Right ventricular hypertrophy: Chronic pulmonary hypertension places increased stress on the right ventricle, causing compensatory hypertrophy.
Decreased diastolic compliance: The hypertrophied right ventricle becomes stiffer, impairing its ability to fill during diastole. This leads to elevated right atrial pressure and systemic venous congestion, manifesting as jugular venous distention and pedal edema.

Incorrect answers:
A. P-wave height reflects right atrial enlargement, which may increase secondary to chronic pulmonary hypertension. However, this does not directly explain the patient’s current symptoms.

B. Increased R-wave height indicates right ventricular hypertrophy, which is a common finding in pulmonary hypertension.

C. Living at high altitude stimulates erythropoiesis, increasing hematocrit.

E. Chronic pulmonary hypertension places increased stress on the right ventricle, causing compensatory hypertrophy and increase in right ventricular wall thickness.

Question 48:

The correct answer is A. Delayed femoral pulses. This 7-year-old boy presents with a significant blood pressure discrepancy between his upper and lower extremities (135/85 mm Hg in the right upper extremity vs.80/40mm Hg in the right lower extremity) and a systolic ejection murmur heard at the left sternal border and left paravertebral intrascapular area. This clinical presentation is highly diagnostic of coarctation of the aorta, a congenital narrowing of the aortic lumen typically located distal to the left subclavian artery. The obstruction to blood flow causes hypertension proximal to the coarctation (upper extremities) and hypoperfusion distal to it, classically manifesting as a brachial-femoral delay and weakened, delayed femoral pulses.

Incorrect answers:
B. Increased jugular venous pressure is a sign of right-sided heart failure or volume overload. While chronic aortic coarctation increases left ventricular afterload and can eventually cause heart failure, it does not typically present with isolated right atrial pressure elevations in an otherwise asymptomatic child.

C. A nontender mass in the inguinal area suggests an inguinal hernia or an undescended testicle. Although cryptorchidism can be associated with certain genetic conditions that also increase coarctation risk (e.g., Turner syndrome), it is not a direct hemodynamic consequence of the aortic narrowing itself.

D. A pulsatile abdominal mass is the classic hallmark of an abdominal aortic aneurysm (AAA). Abdominal aortic aneurysms arise from chronic transmural inflammation and degradation of the extracellular matrix, which typically occurs in older adults with extensive smoking and atherosclerotic histories.

E. Pulsus paradoxus is an exaggerated drop in systemic systolic blood pressure (>10 mm Hg) during inspiration. This clinical finding is characteristically associated with cardiac tamponade, severe asthma, chronic obstructive pulmonary disease (COPD), or constrictive pericarditis rather than localized post-ductal aortic coarctation.

Question 49:

The correct answer is A. This patient most likely has Sarcoidosis, which presents with symptoms like fatigue, constipation, and frequent urination, as well as hypercalcemia (calcium level of 11.1 mg/dL). Sarcoidosis is characterized by bilateral hilar lymphadenopathy and interstitial lung infiltrates on chest x-ray. The hypercalcemia is most commonly due to increased calcitriol production by activated macrophages within the granulomas. For USMLE, the common presentation of sarcoidosis is a middle aged African American woman with cough and bilateral hilar adenopathy on CXR.

Incorrect answers:
B. While local cytokine release due to tumor metastases can lead to hypercalcemia, the patient’s clinical presentation, including chest x-ray findings, is more consistent with Sarcoidosis.

C. Parathyroid hormone-related peptide (PTHrP) secretion typically occurs in paraneoplastic syndromes, such as in squamous cell carcinoma of the lung. This would present with a solitary lung mass, which is not seen here.

D. Increased secretion of parathyroid hormone is typically associated with hyperparathyroidism and could lead to hypercalcemia. However, this patient’s decreased parathyroid hormone levels suggest a different cause of hypercalcemia.

E. Hyperthyroidism can sometimes cause hypercalcemia, but in this patient, there are no classic symptoms of hyperthyroidism (such as weight loss, heat intolerance, or tachycardia), making this an unlikely cause of the hypercalcemia.

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Question 50:

The correct answer is C. This 34-year-old male presents with symptoms consistent with exocrine pancreatic insufficiency (EPI), likely secondary to chronic pancreatitis, as evidenced by his recurrent episodes of pancreatitis and symptoms of malabsorption. The hallmarks of EPI include steatorrhea (oily, foul-smelling stools), diarrhea, weight loss, and nutrient deficiencies due to insufficient production of pancreatic enzymes.

Among the pancreatic enzymes, lipase is the most critical for digesting dietary fats. Patients with EPI are unable to absorb fats properly due to insufficient lipase, resulting in steatorrhea. Therefore, treatment with lipase (as part of pancreatic enzyme replacement therapy) is the most beneficial for this patient.

Incorrect answers:
A. Amylase is responsible for digesting carbohydrates. While carbohydrate digestion may be affected to some extent in EPI, it is not the primary issue. The patient’s symptoms of steatorrhea, weight loss, and malabsorption are due to fat malabsorption, which is primarily caused by lipase deficiency.

B. Lactase is not a pancreatic enzyme; it is produced in the small intestinal brush border and is responsible for digesting lactose, the sugar in dairy products. Lactase deficiency causes lactose intolerance, which presents with bloating, diarrhea, and cramping after dairy consumption. This patient’s symptoms are more consistent with fat malabsorption, not lactose intolerance, so lactase supplementation would not be effective.

D. Proteases (such as trypsin and chymotrypsin) are responsible for digesting proteins. While protease insufficiency can occur in EPI, fat digestion is most severely affected because lipase is more vulnerable to inactivation and degradation. Steatorrhea and malabsorption are primarily due to lipase deficiency, making lipase the priority enzyme for replacement therapy.

E. Tryptase is not a pancreatic enzyme; it is a mast cell-derived enzyme that plays a role in allergic and inflammatory processes. It has no role in digestion.

Question 51:

The correct answer is C. Eradication of intestinal organisms. This 46-year-old man presents with fever, right upper quadrant pain, and a 4-cm hypoechoic liver lesion after traveling to India, confirming an amebic liver abscess caused by Entamoeba histolytica. Treatment requires a dual-drug regimen: a tissue amebicide followed by a luminal amebicide. Metronidazole targets and kills the trophozoites causing the invasive tissue infection within the liver abscess wall and colon wall. However, metronidazole is rapidly absorbed in the upper GI tract and fails to reach therapeutic concentrations within the bowel lumen to eliminate cysts. Non-absorbable paromomycin is added to act strictly as a luminal amebicide to eradicate remaining intestinal cysts and prevent colonization and recurrence.

Incorrect answers:
A. Avoidance of drug resistance development is the rationale behind multi-drug combination therapy for conditions like Mycobacterium tuberculosis or HIV infection, where high mutation rates lead to rapid single-drug resistance. It is not the physiological reason for combining metronidazole and paromomycin in amebiasis.

B. Effective penetration into the liver is the exact therapeutic role of metronidazole, which distributes excellently into body tissues and abscess cavities. Paromomycin is a highly polar aminoglycoside that is completely unabsorbed from the gastrointestinal tract, meaning it achieves zero tissue or liver concentration.

D. Prevention of central nervous system infection would require a lipophilic drug capable of crossing the blood-brain barrier. Extraintestinal amebiasis can rarely cause brain abscesses, but paromomycin remains confined entirely to the lumen of the bowel and provides no systemic or CNS protection.

Question 52:

The correct answer is D. This 3-year-old boy most likely has rhabdomyosarcoma, given the presence of a bulging eye without a history of trauma. Biopsy confirms the diagnosis by identifying striations within muscle cells. Immunohistochemistry can demonstrate expression of desmin

Incorrect answers:
A. Neuroblastoma typically presents as an abdominal mass that may cross the midline and is associated with catecholamine secretion (e.g., hypertension, sweating). While it can metastasize to the orbit, it would usually be accompanied by systemic symptoms such as weight loss or irritability.

B. Pheochromocytoma is a catecholamine-secreting tumor of the adrenal medulla, presenting with episodic hypertension, headaches, palpitations, and tremors. It is rare in children and does not cause unilateral eye bulging.

C. Retinoblastoma presents with leukocoria (white pupillary reflex) rather than proptosis. It is a malignancy of the retina and is not associated with muscle striations on biopsy.

E. Thyroid cancer typically presents as a painless midline neck mass

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Question 53:

The correct answer is E. This patient presents with a sudden onset of fever, shortness of breath, palpitations, tachycardia, and low blood pressure, all of which are indicative of sepsis. The infected wound shows diffuse swelling, erythema, and necrosis, which are characteristic of a Staphylococcus aureus infection. Additionally, the patient’s profession as a carpenter makes him more susceptible to Staphylococcus aureus, as this bacteria commonly infects puncture wounds.

Sepsis is a severe host inflammatory response to an infectious pathogen, which can result in widespread dysfunction of multiple organ systems.

Incorrect answers:
A. Clostridium tetani is usually associated with wounds from rusty nails and presents with opisthotonos (arching back) and trismus (lockjaw) rather than systemic sepsis.

B. Mycobacterium abscessus typically causes chronic infections rather than acute sepsis in immunocompromised patients. The rapid onset of symptoms in this patient suggests an acute bacterial infection, making Staphylococcus aureus the more likely cause.

C. Pasteurella multocida is commonly linked to animal bites or scratches. It typically causes localized cellulitis rather than a severe systemic infection like sepsis, making it an unlikely cause in this patient.

D. Pseudomonas aeruginosa can infect puncture wounds but typically produces a distinct greenish wound discharge. Although it can cause infections, it is less likely to result in sepsis in this situation compared to Staphylococcus aureus.

Question 54:

The correct answer is C. This patient has a spinal cord injury at the T1 level, as indicated by the weakness and atrophy of the intrinsic hand muscles and loss of sensation below the clavicles (which corresponds to the T1 dermatome). The hyperreflexia and increased muscle tone in the lower extremities suggest involvement of the corticospinal tract, a hallmark of upper motor neuron dysfunction. The C5-T1 spinal segments innervate the upper extremity. The lower the spinal level, the more distal the motor function impairment.

Incorrect answers:
A & B. A lesion at C5 (choice A) or C7 (choice B) would likely affect the biceps (C5) or triceps (C7) reflexes, but in this patient, both reflexes are normal, ruling out these levels.

D & E. A lesion at T3 (choice D) or T5 (choice E) would spare the hand muscles, since these levels do not contribute to the brachial plexus. Sensory loss at these levels would also follow the corresponding intercostal dermatomes rather than starting at the clavicles.

Question 55:

The correct answer is D. FEV1: 84%, FVC: 78%, TLC: 78%, DLCO: 92%. This 33-year-old man presents with progressive exertional dyspnea, normal lungs on auscultation, and a significantly elevated body mass index (BMI of 46 kg/m², qualifying as class III extreme obesity). Excess soft tissue mass around the chest wall and abdomen mechanically restricts diaphragmatic excursion and chest wall expansion. This classic extrapulmonary restrictive defect leads to reductions in total lung capacity (TLC < 80%) and forced vital capacity (FVC < 80%). Because airflow is completely unobstructed, the FEV1/FVC ratio remains normal or elevated (>70%). Crucially, because the alveolar-capillary membrane remains healthy and intact, the diffusing capacity for carbon monoxide (DLCO) remains entirely preserved (>80%).

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Question 56:

The correct answer is C. Decreased vaginal rugation. This 67-year-old postmenopausal woman presents with a 6-month history of decreased libido. In the absence of psychosocial stressors or hot flashes, her presentation is indicative of genitourinary syndrome of menopause (vulvovaginal atrophy). The chronic loss of circulating ovarian estrogen leads to thinning of the vaginal epithelium, loss of tissue elasticity, and a reduction in submucosal collagen, which manifests on pelvic examination as a pale, dry vaginal wall with decreased vaginal rugation.

Incorrect answers:
A. Bulging of the anterior vaginal wall through the introitus indicates a cystocele, which occurs when the bladder prolapses into the vagina due to pelvic floor muscle weakness, presenting with pelvic pressure or urinary symptoms.

B. Bulging of the posterior vaginal wall through the introitus describes a rectocele, resulting from defects in the rectovaginal septum allowing the rectum to herniate forward, presenting with defecatory dysfunction.

D. Palpable ovaries bilaterally on pelvic examination is abnormal in a 67-year-old woman. Ovaries naturally undergo postmenopausal atrophy and regression, making them unpalpable; palpable ovaries in this age group warrant evaluation for malignancy.

E. A uterine size of 8 cm is expected for a premenopausal, multiparous woman. Following menopause, the lack of estrogen signaling leads to physiological atrophy, reducing the postmenopausal uterine length to less than 3-5 cm.

Question 57:

The correct answer is E. “What is your understanding of what your hemoglobin A1c should be?”. This 47-year-old woman with type 1 diabetes presents with a poorly controlled hemoglobinA1c of 12 %. Her statement indicates frustration and potential gaps in medical literacy regarding her target glycemic goals. When addressing chronic disease mismanagement, the initial response should be open-ended and non-judgmental. Assessing the patient’s baseline knowledge identifies specific educational deficits and builds therapeutic rapport before introducing lifestyle or medical adjustments.

Incorrect answers:
A. Asking “Do you ever skip a dose of insulin?” is a closed-ended, leading question. This phrasing can feel accusatory, causing the patient to become defensive or give socially desirable answers rather than sharing true adherence barriers.

B. Stating “Noncompliance in your disease can be deadly” uses an aggressive approach. Frightening a patient who is already expressing difficulty lowers self-efficacy and alienates them from continuing medical follow-up.

C. Asking for “recent traumas or stresses that would be contributing to your poor compliance” prematurely assumes that poor adherence is driven by psychological trauma and utilizes a paternalistic term that damages rapport.

D. Asking “Tell me about the kinds of foods you eat” focuses too early on a specific lifestyle modification. While nutrition is important, it bypasses exploration of her understanding of the clinical data.

Question 58:

The correct answer is D. A Type II error (also known as a β error) occurs when the null hypothesis is not rejected, even though it is actually false. In other words, it happens when a true effect is present, but the study fails to detect it. By increasing the sample size, the statistical power(1-B) of the study is improved, which means the study is more likely to correctly detect a real effect if it exists. This increase in power(1-B) reduces the probability of making a Type II error (β error). Increasing Sample size leads to increased power while decreasing alpha error leads to increased confidence level.

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Question 59:

The correct answer is D. In cases of urinary tract obstruction, the proximal portion of the affected segment becomes dilated, while the distal portion typically exhibits normal or decreased flow. In this patient, the MRI findings indicate dilation of the renal pelvis without dilation of the ureter, suggesting that the obstruction is at the ureteropelvic junction (UPJ).

Distension or dilation of the renal pelvis, known as hydronephrosis, is typically caused by obstruction from renal stones, benign prostatic hyperplasia (BPH), cervical cancer, or ureteral injury. It can harm renal tissue, leading to atrophy of the renal cortex and medulla.

Incorrect answers:
A. Obstruction at the bladder neck can lead to bilateral hydronephrosis since it impacts the outflow from both kidneys, rather than just one.

B. Mid-ureter obstruction would lead to dilation of both the upper ureter and the renal pelvis, which is not observed here.

C. Since only the renal pelvis is dilated and there is no dilation of the ureters, this rules out a general obstruction of the renal pelvis.

E. Obstruction at the ureterovesical junction would cause dilation of the distal ureter and renal pelvis, which is not the case in this patient.

Question 60:

The correct answer is G. Myofibroblasts. This 53-year-old woman presents with an enlarging, 2-cm subcutaneous mass developing directly at the site of a previous surgical resection. The histopathologic images show a dense, disorganized proliferation of thick, hyalinized collagen bundles (collagen type I and III) within the dermis. This presentation describes a keloid, which results from an abnormal wound healing response that extends beyond the original borders of injury. The excessive extracellular matrix deposition is driven by the overactivation and persistent differentiation of myofibroblasts, stimulated by elevated local growth factors such as transforming growth factor-beta (TGF beta).

Incorrect answers:
A. Macrophages are phagocytic cells that clear wound debris and release regulatory cytokines rather than structurally proliferating or depositing the fibrous collagen matrix.

B. Mast cells are tissue-resident granulocytes containing histamine that mediate acute allergic reactions and are not the primary cellular component seen in keloid tissue.

C. Melanocytic cells are epidermal cells that produce melanin, meaning their benign or malignant proliferation forms skin nevi or melanomas instead of dense dermal collagen bundles.

D. Mesothelial cells are simple squamous epithelial cells that line serosal cavities like the pleura and do not reside in or contribute to cutaneous wound healing.

E. Myeloblasts are bone marrow stem cell precursors to granulocytes whose malignant proliferation causes acute myeloid leukemia rather than localized solid fibrous tumors.

F. Myocytes are contractile muscle cells, meaning smooth muscle cell proliferations (leiomyomas) present as sweeping cellular fascicles with elongated nuclei rather than acellular collagen bundles.

Block 4

Question 61:

The correct answer is E. Dysfunctional interferon-gamma receptors. This 13-year-old boy presents with left ischial osteomyelitis and a right knee effusion caused by Mycobacterium avium complex (MAC) in the setting of a negative HIV test. MAC is a low-virulence, non-tuberculous environmental organism that rarely causes invasive disease in immunocompetent individuals. Clearance of intracellular mycobacteria relies strictly on the IL-12 / Interferon-gamma (IFN gamma) signaling pathway between macrophages and Th1 helper T cells. A congenital defect resulting in dysfunctional interferon gamma receptors prevents macrophage activation, rendering the patient highly susceptible to severe, disseminated infections with atypical, opportunistic mycobacteria.

Incorrect answers:
A. An absent expression of C5 disrupts the formation of the membrane attack complex (C5b-C9), which characteristically predisposes patients to recurrent, disseminated infections with Neisseria species rather than intracellular mycobacteria.

B. Bilobed neutrophil nuclei with abnormal granule morphology describe Chédiak-Higashi syndrome, an autosomal recessive lysosomal trafficking defect that presents with oculocutaneous albinism, peripheral neuropathy, and recurrent pyogenic infections.

C. A decreased absolute CD8+ T-lymphocyte count impairs cellular cytotoxicity against viral infections and tumor cells but does not match the specific isolated susceptibility to atypical environmental mycobacteria seen with IFN gamma axis defects.

D. A decreased serum concentration of IgG is the hallmark of humoral immunodeficiencies, such as X-linked agammaglobulinemia or CVID, which characteristically present with recurrent sinopulmonary infections caused by encapsulated extracellular bacteria.

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Question 62:

The correct answer is C. The fetal abnormalities in this case—misshapen cranium, pericardial effusion, and echogenic bowel—are characteristic findings of Potter sequence, a complication of oligohydramnios. Oligohydramnios typically results from defects in fetal urine production, most commonly due to renal abnormalities. Additionally, this patient has a history of lisinopril use during pregnancy, which is known to cause fetal kidney damage, particularly when taken during the second and third trimesters.

Oligohydramnios is a condition defined by inadequate amniotic fluid. It can result from bilateral renal agenesis, placental insufficiency, or posterior urethral valves, especially in male fetuses.

An important complication of oligohydramnios is Potter sequence. The characteristics of Potter sequence include:

Pulmonary hypoplasia caused by limited fetal lung expansion.
Abnormalities in limbs caused by restricted fetal movement
Facial features like a flat face and low-set ears

Incorrect answers:
A & B. These conditions can lead to respiratory distress in the fetus, but they are not involved in Potter sequence.

D. Placental perfusion defects can cause placental insufficiency, which may lead to oligohydramnios, but in this case, the history of lisinopril use makes renal dysfunction the more likely cause.

E. Maternal prostaglandin synthesis primarily affects fetal lung development and does not play a role in oligohydramnios.

Question 63:

The correct answer is C. Massive myocardial infarction. This 54-year-old woman dies 30 minutes after the sudden onset of severe respiratory distress. The autopsy photomicrograph of the lung tissue shows engorged, congested alveolar capillaries along with alveoli filled with homogeneous, pale pink intra-alveolar transudate, diagnostic of acute pulmonary edema. Left ventricular failure from a massive myocardial infarction acutely increases pulmonary venous hydrostatic pressure, causing rapid alveolar flooding and sudden death.

Incorrect answers:
A & E. Bilateral bronchopneumonia and viral pneumonia present with prominent cellular infiltrates, showing dense intra-alveolar neutrophils or interstitial lymphocytes rather than acellular transudative fluid.

B. Bronchial asthma demonstrates airway-specific changes including smooth muscle hypertrophy, mucous plugging with Curschmann spirals, and eosinophilic infiltration instead of isolated alveolar fluid.

D. A thromboembolus in a bronchial artery is rare and does not cause diffuse edema, whereas a pulmonary embolism occludes the pulmonary artery without producing immediate transudative alveolar flooding.

Question 64:

The correct answer is E. This patient is recently treated for mycobacterium with ciprofloxacin and rifampin. Doubling the dose of warfarin would typically result in a higher INR value; however, the metabolism of warfarin has been accelerated due to rifampin inducing the P450 system, leading to decreased plasma concentrations and subtherapeutic INR levels. Medications that inhibit P450 system: Cimetidine, Ciprofloxacin, Omeprazole, Ketoconazole, Erythromycin, Protease Inhibitors and Grapefruits. (COKE, PIe and Grapefruits).

Incorrect answers:
A. Decreased protein binding would result in an increased free concentration of warfarin, leading to a supratherapeutic INR (increased bleeding time).

B. Eradication of gut flora reduces vitamin K synthesis, which would lead to an increased INR, not a subtherapeutic level as seen in this patient.

C. The patient’s alcohol intake remains stable (two 12-oz beers daily), making it unlikely to contribute to the current findings.

D. Increased consumption of vitamin K-rich vegetables (e.g., spinach) can decrease warfarin efficacy and result in subtherapeutic INR levels; however, this would typically affect baseline INR values and not require doubling the warfarin dose as seen in this patient.

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Question 65:

The correct answer is A. Age. This 80-year-old woman presents with a 5-month history of progressive, painless reduction in central vision, specifically affecting reading and driving, along with a requirement for brighter light. The funduscopic photograph reveals numerous scattered, yellowish-white subretinal deposits concentrated around the macula. These findings are classic for soft drusen, diagnostic of dry age-related macular degeneration (AMD). Advanced age (typically >50 years, with risk increasing exponentially each decade) is the single most significant, non-modifiable risk factor for the development and progression of AMD due to chronic oxidative stress and accumulation of lipofuscin within the retinal pigment epithelium.

Incorrect answers:
B. Alcohol consumption is not an established, significant risk factor for the development of age-related macular degeneration.

C. Ethnicity is a known risk factor, but AMD is actually significantly more prevalent in White individuals than in African American individuals.

D. Family history of type 2 diabetes or glaucoma does not increase AMD risk, though a direct family history of AMD itself is a known risk factor.

E. Hypertension is a cardiovascular risk factor that can contribute to retinal vascular diseases but is less statistically significant than advanced age for AMD.

Question 66:

The correct answer is E. This 4-year-old boy, undergoing induction chemotherapy for acute lymphoblastic leukemia (ALL), presents with fever, pallor, and signs of infection. These findings strongly suggest neutropenia, a common complication of chemotherapy, as the most likely underlying mechanism of his susceptibility to infection.

Neutropenia is a condition characterized by an abnormally low absolute neutrophil count (ANC), which predisposes patients to infections, particularly from:

Gram-negative bacteria (e.g., Escherichia coli, Pseudomonas aeruginosa).
Gram-positive bacteria.
Fungi (e.g., Candida, Aspergillus).

This patient’s fever and positive blood cultures in the setting of chemotherapy and a central venous catheter strongly indicate neutropenic fever, which is a medical emergency requiring immediate broad-spectrum antibiotic therapy to prevent sepsis. The patient may presents with mild symptoms as mild fever due to a lack of neutrophil-mediated inflammation.

Incorrect answers:
A. Complement deficiencies, such as C5-C9 deficiency, increase susceptibility to Neisseria infections, not gram-negative rods. Also, it is not the problem in chemotherapy.

B. Hypogammaglobulinemia is associated with recurrent bacterial infections, especially encapsulated organisms (e.g., Streptococcus pneumoniae, Haemophilus influenzae). While this can occur in certain leukemias, it does not explain the acute gram-negative bacteremia seen here.

C. T-cell dysfunction increases the risk of opportunistic infections (e.g., viruses, fungi, intracellular bacteria). This patient’s presentation is more consistent with neutropenic fever.

D. Anti-TNF therapy (e.g., infliximab) is associated with reactivation of latent infections like tuberculosis, not gram-negative rod bacteremia.

Question 67:

The correct answer is A. To calculate absolute risk reduction, we first determine the absolute risks for both procedures and then calculate their difference. There were 18 cases per 1,000 women for Procedure B, making the absolute risk of wound infection from Procedure B 18/1,000. Likewise, the absolute risk of wound infection for Procedure A is 12/1,000. By finding the difference between these two absolute risks, we obtain the absolute risk reduction.

Incorrect answers:
B, C & E. These involve incorrect calculations for absolute risk reduction.

D. While calculating absolute risk or absolute risk reduction, we only consider patients enrolled in the study. The total number of patients (including those not enrolled) is not relevant to this calculation.

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Question 68:

The correct answer is B. Acute myelogenous leukemia. This 67-year-old man presents with severe systemic symptoms (fever, fatigue), signs of bone marrow suppression (anemia, profound thrombocytopenia causing bleeding gums and ecchymoses), and an elevated leukocyte count (24,500mm). The peripheral blood smear photomicrograph shows numerous large, immature myeloid blasts with abundant cytoplasm, fine chromatin, prominent nucleoli, and diagnostic Auer rods (linear crystalline inclusions formed by the fusion of azurophilic granules). These clinical and cytomorphological findings are highly diagnostic of Acute Myelogenous Leukemia (AML).

Incorrect answers:
A. Acute lymphocytic leukemia is characterized by a proliferation of lymphoblasts, which typically show a high nuclear-to-cytoplasmic ratio and condensed chromatin, but they never contain Auer rods or cytoplasmic azurophilic granules.

C. Chronic lymphocytic leukemia manifests as a neoplastic proliferation of mature, incompetent B lymphocytes that characteristically display fragile membranes, appearing on a peripheral blood smear as crushed smudge cells rather than large immature blasts.

D. Chronic myelogenous leukemia is a myeloproliferative neoplasm driven by the t(9;22) translocation that presents with a full spectrum of maturing granulocytic cells (neutrophils, metamyelocytes, myelocytes, promyelocytes) and low blast percentages (<20%) in the chronic phase.

Question 69:

The correct answer is D. This patient most likely has familial hypercholesterolemia supported by the presence of tendinous xanthomas, high cholesterol (410 mg/dl), and early-onset myocardial infarction. Family history also supports the diagnosis. LDL receptor deficiency leads to decreased clearance of LDL cholesterol, resulting in hypercholesterolemia. Medications targeted to decrease LDL receptor recycling can help treat this condition.

Incorrect answers:
A. Apo B48 is associated with abetalipoproteinemia, which is diagnosed in infancy. It’s characterized by the presence of steatorrhea and deficiency of fat-soluble vitamins. Lab value shows low VLDL, LDL, chylomicrons and cholesterol

B & E. Deficiency of Apo C and LPL activity are associated with hypertriglyceridemia and pancreatitis, not hypercholesterolemia.

C. Reduced HMG-CoA reductase activity is associated with low levels of cholesterol. Statins work by inhibiting this enzyme to lower cholesterol, which contradicts the findings in this patient.

Question 70:

The correct answer is C. Delayed maturation of detrusor and sphincter control. This 5-year-old boy presents with primary nocturnal enuresis, defined as involuntary urinary incontinence at night in a child aged 5 years or more who has never achieved a 6-month period of nighttime dryness. The patient has a completely normal physical examination, normal growth parameters, and a negative urinalysis, which rules out secondary medical causes. The underlying pathophysiology is a delayed physiological maturation of detrusor muscle stability and urethral sphincter control during sleep, often coupled with an underdeveloped nocturnal arousal response to bladder fullness or a transiently low nighttime secretion of antidiuretic hormone (ADH).

Incorrect answers:
A & E. An abnormal insertion of the terminal ureter (ectopic ureter) and retrograde flow of urine (vesicoureteral reflux) are structural anatomical anomalies that cause continuous, daytime and nighttime dampening or recurrent febrile urinary tract infections rather than isolated nighttime bed-wetting.

B. Bladder colonization leading to recurrent urinary tract infection would present with acute symptoms such as dysuria, urinary frequency, urgency, or suprapubic pain, alongside an abnormal dipstick urinalysis showing positive leukocytes or nitrites.

D. An occult neurologic lesion leading to disruption of bladder innervation, such as spina bifida occulta, would manifest with lower extremity neurological deficits, abnormal lower extremity reflexes, gait changes, or cutaneous spinal tufts of hair rather than an isolated nocturnal symptom.

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Question 71:

The correct answer is A. The key clinical features in this patient are hard stools (described as “pebbles”), hypoactive bowel sounds, and a history of infrequent bowel movements (last one 5 days ago). A stool pattern of fewer than 3 bowel movements per week with hard stools strongly points to constipation. The bright red blood on the tissue and the rectal mucosa protrusion suggest the presence of an anal fissure, which is a common complication of constipation. While constipation is the main issue here, the patient doesn’t show any other specific signs pointing to any distinct condition.

Constipation is a common gastrointestinal problem that can occur at any age. Some conditions associated with constipation in children include Hirschsprung disease, volvulus, intussusception, cystic fibrosis, drug-induced and meconium ileus.

Incorrect answers:
B. While cystic fibrosis can cause constipation, it usually presents with other hallmark features like salty sweat, recurrent respiratory infections, failure to thrive, and pancreatic insufficiency. This patient lacks these symptoms, making cystic fibrosis unlikely.

C. Hirschsprung disease can lead to constipation. Still, it is typically diagnosed in infants with failure to pass meconium and signs of intestinal obstruction, such as abdominal distension and bilious vomiting. The absence of these features in this case, makes this diagnosis unlikely.

D. Hookworm infections typically cause bloody diarrhea, but this patient has hard stools, which indicates constipation. Absence of travel history also makes it unlikely.

E. Intussusception usually presents with colicky abdominal pain, vomiting, and currant jelly stools instead of pebble like hard stools.

Question 72:

The correct answer is D. 2 of 3. This 27-year-old nulligravid woman (individual III-5, indicated by the arrow) seeks preconception counseling regarding her carrier risk for cystic fibrosis (CF), an autosomal recessive disorder. Her full brother (individual III-6) is affected, meaning he has the genotype aa. Since their parents (II-3 and II-4) are phenotypically unaffected but have produced an affected child, they must both be obligate heterozygous carriers (Aa x Aa).

According to Mendelian inheritance rules, a cross between two carriers yields the following genotypic probabilities for their offspring:

25% chance of being unaffected homozygous (AA)
50% chance of being an unaffected heterozygous carrier (Aa)
25% chance of being affected homozygous (aa)

Because individual III-5 is explicitly observed to be phenotypically unaffected, her possibility of having the aa genotype is completely eliminated from the denominator. This leaves her with 3 possible genotypes (AA, Aa, or aA), 2 of which are carriers (Aa or aA). Therefore, her conditional probability of being a carrier is 2 of 3.

Question 73:

The correct answer is E. alpha Adrenergic agonist. This 45-year-old woman presents with an acute myocardial infarction (inferior and posterior STEMI) complicated by profound hypotension (70 mm Hg systolic) and tachycardia (136 /min), diagnostic of cardiogenic shock. The therapeutic goal is to rapidly increase her systemic blood pressure to maintain coronary and vital organ perfusion without further escalating her heart rate, which would dangerously increase myocardial oxygen demand. Phenylephrine is a pure alpha adrenergic agonist that causes prominent vasoconstriction to increase systemic vascular resistance and blood pressure. Because it lacks beta1 agonist activity, it achieves this without directly increasing her pulse.

Incorrect answers:
A & B. Dopamine agonists and beta-adrenergic agonists (such as dobutamine or isoproterenol) possess significant beta 1 stimulatory properties that drastically elevate heart rate and myocardial contractility, risking severe arrhythmias or expansion of the infarct area.

C. beta Adrenergic antagonists (beta blockers) decrease heart rate and contractility, meaning their administration during acute cardiogenic shock will further collapse cardiac output and exacerbate circulatory failure.

D. alpha Adrenergic antagonists cause systemic vasodilation and drop total peripheral resistance, which would critically worsen this patient’s pre-existing profound hypotension.

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Question 74:

The correct answer is D. This patient has a Horseshoe kidney, evidenced by the ultrasonic finding and history of recurrent UTIs. It occurs due to the fusion of the inferior poles of the metanephros during embryological development, leading to abnormal kidney positioning. Horseshoe kidney is a common finding with Turner syndrome. It increases the risk of UTIs.

Incorrect answers:
A. While kidney ascension and rotation are important for normal kidney development; they do not contribute to the formation of horseshoe kidney.

B. Similarly, to option A, ascension and rotation are essential for normal kidney development, but they are not responsible for the fusion seen in horseshoe kidney.

C. Failure of the ureteric bud to develop results in agenesis or absence of the kidney, not horseshoe kidney. The ureteric bud plays a key role in kidney development through inductive signaling.

Question 75:

The correct answer is C. “I’m glad you came in today and told me about this.”. This 16-year-old boy presents with a 3-day history of a scrotal “pimple” and expresses significant anxiety about having a sexually transmitted infection (STI). Adolescents often face substantial barriers, fear, and stigma when seeking care for reproductive or sexual health concerns. The most appropriate initial response by the physician must prioritize establishing a safe, non-judgmental environment. Validating the patient’s initiative and normalizing the visit builds immediate therapeutic rapport, which encourages transparent disclosure before proceeding to clinical or sexual history gathering.

Incorrect answers:
A, D & E. Asking about partner gender, protection use, or the timing of the last sexual experience are essential parts of a comprehensive sexual history (the 5 Ps), but jumping directly into these intrusive, closed-ended questions before validating the patient’s anxiety can feel interrogative and shut down communication.

B. Asking if his mother knows he is sexually active breaches the expectation of adolescent confidentiality regarding sexual health, introducing fear of parental disclosure that could cause the patient to withhold information or abandon care altogether.

Question 76:

The correct answer is D. This patient has features of insulin resistance (elevated fasting glucose and BMI), which is common in metabolic syndrome and prediabetes. As a result, her body compensates by producing more insulin to overcome the resistance in tissues like adipose and muscle. Therefore, her insulin levels are likely to be elevated. In early DM, insulin level is high to overcome high glucose. Later, Pancreas gets exhausted and low insulin levels ensue.

Incorrect answers:
A. Insulin resistance and metabolic syndrome are generally associated with increased cholesterol production and altered lipid profiles, NOT decreased cholesterol excretion. Elevated insulin levels promote lipid storage and increase hepatic lipogenesis.

B. Increased adipose tissue, especially in postmenopausal women, contributes to the conversion of androstenedione to estrone.

E. Increased growth hormone levels are typically associated with acromegaly, a condition characterized by abnormal growth of bones and soft tissues, leading to enlarged hands, feet, and facial features.

F. Elevated TSH levels are indicative of primary hypothyroidism, which can lead to weight gain. However, this patient’s weight gain is more likely related to insulin resistance and metabolic syndrome rather than thyroid dysfunction.

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Question 77:

The correct answer is D. This patient has a history of episodes of full-body weakness (paralysis) triggered by exercise (football) and eating high-carbohydrate meals like pasta and salty foods. These episodes are temporary, fitting the pattern in hypokalemic periodic paralysis. This rare condition is characterized by muscle weakness or paralysis that often occurs after exercise or a large carbohydrate meal, making it the most likely diagnosis.

Incorrect answers:
A. Calcium imbalances can lead to muscle issues like tetanic contractions in hypocalcemia; however, this patient experiences self-resolving episodes triggered by exercise, which aligns more closely with hypokalemic periodic paralysis rather than calcium-related problems.

B. Chloride imbalances can affect acid-base balance, causing hyperchloremic acidosis. This condition can lead to muscle weakness and breathing difficulties. However, these symptoms don’t quite match those of this patient with exercise-triggered paralysis.

C. Magnesium deficiency can certainly lead to muscle weakness, twitching, and even seizures. However, magnesium-related problems typically do not present with the same pattern of temporary paralysis after exercise and meals that this patient is experiencing.

E. Sodium imbalances, whether too low or too high, can lead to issues such as confusion, muscle twitching, and weakness. However, the episodes described by the patient don’t seem to align with sodium-related problems.

Question 78:

The correct answer is F. This patient presents with a 1-week history of increased urination and excessive thirst, along with a low urine osmolality that corrects after administration of desmopressin (an ADH analogue). These findings are consistent with central diabetes insipidus (DI), which is caused by decreased production of ADH. The supraoptic nucleus of the hypothalamus is responsible for synthesizing ADH (vasopressin). Vasopressin test is used to differentiate between Central vs nephrogenic DI.

Incorrect answers:
A. The anterior pituitary gland secretes hormones such as GH, prolactin, ACTH, TSH, FSH, and LH. Dysfunction here typically presents with signs like fatigue, failure to lactate.

B. Damage to Bowman space this structure would result in glomerular diseases, such as glomerulonephritis, characterized by proteinuria, hematuria, and renal dysfunction, which are not present here.

C. Damage to the glomerulus, as seen in glomerulonephritis, leads to proteinuria, hematuria, and renal impairment. This patient does not have such findings.

D. The hypophysial portal system is a network of blood vessels connecting the hypothalamus to the anterior pituitary and is not involved in ADH production or release.

E. The loop of Henle is primarily involved in concentrating urine via the Na/K/2Cl co-transporter. Genetic abnormalities of this channel can cause Bartter syndrome, which mimics the effects of loop diuretics. However, Bartter syndrome would not respond to ADH administration.

Question 79:

The correct answer is B. Seminoma. This 25-year-old man presents with a painless, firm 3-cm nodule in his right testicle, and ultrasonography confirms a solid mass, which is highly indicative of a testicular germ cell tumor. Microscopic examination revealing uniform round cells with clear cytoplasm (due to high glycogen content) and prominent, centrally placed nuclei organized into lobules separated by fibrous septa is the classic histologic presentation of a seminoma. Seminomas are the most common pure testicular germ cell tumor in young adults and characteristically exhibit this “fried-egg” cellular appearance.

Incorrect answers:
A. Embryonal carcinoma presents histologically as a highly pleomorphic, aggressive, and poorly differentiated neoplasm forming primitive glandular, alveolar, or papillary structures with extensive areas of hemorrhage and necrosis.

C. Sertoli cell tumors are rare sex cord-stromal tumors that present with well-defined tubules lined by columnar cells, often clinically silent but capable of secreting estrogens.

D. Teratomas contain a disorganized mixture of mature or immature tissues derived from multiple germ layers, showing structures like cartilage, adipose tissue, neural elements, or squamous epithelium.

E. Yolk sac tumors are the most common testicular tumor in children under 3 and are characterized histologically by a lace-like reticular network, intracellular alpha-fetoprotein (AFP) globules, and primitive glomerulus-like structures known as Schiller-Duval bodies.

Question 80:

The correct answer is D. This 2-week-old male newborn was exposed to chronic maternal alcohol consumption during pregnancy, which is consistent with fetal alcohol syndrome (FAS). A key physical finding in FAS is short palpebral fissures and other characteristic craniofacial abnormalities. Alcohol disrupts cellular processes such as neural crest migration, leading to the characteristic features of FAS. Key findings include:

Craniofacial abnormalities:

Short palpebral fissures (the horizontal length of the eye openings is reduced).
Smooth philtrum (flattened groove between the nose and upper lip).
Thin vermillion border (upper lip).

Growth retardation:

Prenatal or postnatal growth deficiency.

Neurological abnormalities:

Cognitive impairment.
Behavioral issues, such as hyperactivity or learning disabilities.

Incorrect answers:
A. This is a congenital malformation of the urethral opening and is unrelated to alcohol exposure.

B. Limb hypoplasia is not characteristic of FAS.

C. Neck webbing is a feature of Turner syndrome (45, X) and is not associated with prenatal alcohol exposure.

E. While FAS may result in central nervous system abnormalities, spasticity is not a characteristic feature.

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Block 5

Question 81:

The correct answer is D. This 25-year-old woman presents with symptoms of hyperthyroidism (palpitations, heat intolerance, moist palms, bilateral lid lag) and laboratory findings of undetectable TSH with elevated free T₄ and T₃ concentrations. Her symptoms and recent postpartum status suggest postpartum thyroiditis, a transient form of thyroid dysfunction caused by lymphocytic infiltration of the thyroid.

Postpartum thyroiditis is a form of subacute lymphocytic (painless) thyroiditis that occurs within 12 months after delivery. It is caused by an autoimmune attack on thyroid tissue, leading to the release of preformed thyroid hormones into the circulation (thyrotoxicosis phase). The condition often progresses through three phases:

Thyrotoxicosis phase: Increased release of thyroid hormones due to inflammation and damage to thyroid follicles.
Hypothyroidism phase: Following depletion of thyroid hormone stores, patients may develop transient hypothyroidism.
Euthyroid phase: The thyroid eventually recovers to normal function in most cases.

Incorrect answers:
A. This occurs in toxic adenomas or toxic multinodular goiter. These conditions are not associated with postpartum thyroiditis or recent pregnancy.

B. This is a marker of thyroid inflammation but does not cause the signs and symptoms seen in this patient.

C. Hypothalamic ischemia, such as in Sheehan syndrome, leads to hypopituitarism and hypothyroidism, not hyperthyroidism.

E. TSH receptor autoantibodies are found in Graves’ disease, which causes hyperthyroidism through increased thyroid hormone synthesis. This patient’s thyroiditis is due to destruction and release of preformed hormones, not increased synthesis.

Question 82:

The correct answer is D. To solve this question, we focus on false positives—cases where the test is positive in individuals without the disease. Let’s break down the data:

1. Key Data from the Question:

The screening test was administered to:

1000 women with breast cancer
1000 women without breast cancer.

Results:

The test correctly identifies 250 of the subjects with breast cancer as positive.
The test identifies 100 of the subjects without breast cancer as positive (false positives).

2. False Positive Rate (FPR):

The false positive rate is the proportion of non-diseased individuals incorrectly identified as positive:

False Positives / (Total non-diseased individuals) = 100 / 1000 = 0.1

3. Population Characteristics:

The test is applied to a population of 100,000 women.
Prevalence of breast cancer: 80 per 100,000.
○ Number of women without breast cancer = 100,000 – 80 = 99,920

4. Calculate False Positives:

Using the false positive rate and the total number of non-diseased individuals:

Probability of a false positive * Non-diseased individuals ….

FPR * Non-diseased individuals = 0.1 * 99,920 = 9,920

Thus, the expected number of false positives in this population is 9,992.

Question 83:

The correct answer is E. This 73-year-old man presents with symptoms of left hemispatial neglect, evidenced by his tendency to ignore stimuli on the left side (walking into the left side of a doorframe, tripping over a chair on his left side, and only perceiving right-sided tactile stimuli when bilateral stimuli are applied). The lesion most likely involves the right parietal lobe, which is responsible for spatial attention and awareness.

The parietal lobe plays a critical role in spatial processing and attention. A lesion in the right parietal lobe can result in contralateral hemispatial neglect, causing the patient to:

1. Ignore the left side of space: The patient fails to perceive or respond to stimuli on the left side.
2. Exhibit tactile extinction: When simultaneous tactile stimuli are applied to both sides, the patient only perceives the right-side stimulus.
3. Display visual field neglect: The left lower quadrantanopia suggests a partial visual field defect, consistent with parietal lobe involvement.

The absence of motor or language deficits and preserved proprioception further localize the lesion to the parietal lobe, rather than other brain regions such as the frontal or temporal lobes.

Incorrect answers:
A & D. Lesions in the frontal lobe typically cause motor deficits or language impairments (e.g., Broca’s aphasia), not spatial neglect.

B. A lesion in the left parietal would affect spatial attention primarily on the right side, not the left.

C & F. Lesions in the temporal lobe can cause language deficits (if dominant hemisphere) or memory issues but are not responsible for hemispatial neglect.

Question 84:

The correct answer is E. 22. This female newborn presents with dysmorphic facial features (ocular hypertelorism, short philtrum, micrognathia), a severe conotruncal cardiac anomaly (interrupted aortic arch), and hypocalcemic tetany. This constellation of clinical signs matches a microdeletion syndrome caused by abnormal development of the third and fourth pharyngeal pouches, resulting in thymic hypoplasia and parathyroid hypoplasia. The underlying etiology is a microdeletion on chromosome 22 (specifically 22q11.2), diagnostic of DiGeorge syndrome.

Incorrect answers:
A, C & D. Chromosomes 13, 18, and 21 are associated with major trisomy syndromes (Patau, Edwards, and Down syndromes, respectively) which present with distinct structural defects (e.g., midline cleft lip/palate or holoprosencephaly in trisomy 13; clenched fists and rocker-bottom feet in trisomy 18) rather than isolated 3rd/4th pharyngeal pouch dysgenesis.

B. Chromosome 14 is a large acrocentric chromosome that can be involved in Robertsonian translocations (e.g., contributing to translocation Down syndrome) but microdeletions or defects here do not cause conotruncal heart defects or neonatal hypocalcemic tetany.

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Question 85:

The correct answer is E. Right atrium. This 75-year-old man with a history of alcohol use disorder presents with a 2-month history of palpitations, lightheadedness, fatigue, and shortness of breath. The ECG tracing reveals a regularly irregular rhythm (or regularly regular rhythm depending on the conduction ratio) with a classic “sawtooth” pattern of atrial activation. This presentation is highly diagnostic of atrial flutter. Typical atrial flutter is caused by a large macro-reentrant circuit that propagates around the tricuspid valve annulus within the right atrium, specifically requiring the cavotricuspid isthmus as a critical zone of slow conduction.

Incorrect answers:
A, B & D. The atrioventricular node, bundle of His, and Purkinje fibers constitute the ventricular conduction system; pacing or defects here lead to narrow escape rhythms or heart blocks rather than a macro-reentrant atrial tachyarrhythmia.

C. The left ventricle handles ventricular pacing and arrhythmias; a ventricular focus would manifest as wide, bizarre QRS complexes (such as ventricular tachycardia) accompanied by immediate, severe hemodynamic collapse.

F. The sinoatrial node is the intrinsic pacemaker of the heart; abnormal automaticity here produces sinus tachycardia, which preserves a normal, flat isoelectric baseline between distinct, uniform P waves.

Question 86:

The correct answer is D. Inhibition of cyclooxygenase-1. This 52-year-old woman presents with hematemesis (vomiting blood), hypotension (90/50mm Hg), tachycardia (120/min), and epigastric tenderness following chronic use of oral ibuprofen for knee osteoarthritis. Ibuprofen is a non-selective NSAID that non-selectively inhibits both COX-1 and COX-2 enzymes. Inhibition of cyclooxygenase-1 (COX-1) breaks down the synthesis of mucosal prostaglandins (PGE2 and PGI2), which are essential for maintaining the gastric mucosal barrier. This loss reduces gastric bicarbonate and mucus secretion, increases gastric acid output, and decreases mucosal blood flow, directly predisposing the patient to erosive gastritis, peptic ulceration, and acute upper gastrointestinal bleeding.

Incorrect answers:
A & B. Generation of nitric oxide and increased prostaglandin production are physiologic mechanisms that protect the gastric mucosa; NSAIDs suppress these pathways rather than promoting them.

C. Induction of pro-inflammatory cytokines occurs in infectious or inflammatory states (like H. pylori infection) but is not the primary biochemical mechanism by which systemic NSAIDs disrupt gastric mucosal integrity.

E. Pre-absorptive topical injury to gastric epithelial cells refers to the local, direct acidic irritation caused by certain un-ionized NSAIDs in the stomach lumen. However, the patient’s presentation is primarily driven by the systemic, pharmacodynamic effect of COX-1 inhibition, as evidenced by the concurrent use of topical diclofenac alongside her oral systemic ibuprofen.

Question 87:

The correct answer is A. Deformation. This male newborn presents with short feet that point downwards and are turned inwards at the ankle (clubfoot/talipes equinovarus) following a pregnancy complicated by oligohydramnios (decreased amniotic fluid) due to uteroplacental insufficiency. A deformation is an extrinsic disturbance or mechanical force that structurally alters an otherwise normally developing tissue or organ system. In this patient, the severe reduction of amniotic fluid takes away the protective fluid cushion, resulting in intrauterine crowding. The prolonged, abnormal uterine compression physically forces the fetal limbs into an unnatural position, causing the musculoskeletal deformation.

Incorrect answers:
B. A disruption is an extrinsic breakdown or destruction of a tissue structure that was initially developing entirely normally (e.g., amniotic bands wrapping around a fetal digit or limb, compromising blood supply and causing auto-amputation).

C. A dysplasia is an intrinsic, abnormal cellular organization or architectural arrangement within a specific tissue type during its histogenesis (e.g., osteogenesis imperfecta or skeletal dysplasias).

D. A malformation is an intrinsic, primary error in the initial morphogenic blueprint of an organ or tissue during early embryonic development, completely independent of external physical forces (e.g., congenital heart defects, cleft lip, or neural tube defects).

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Question 88:

The correct answer is B. This patient most likely has a pneumothorax due to the knife wound to the chest. Tachycardia and tachypnea are common in this condition. A large pneumothorax can shift the mediastinum to the opposite side. On physical examination, you would find decreased tactile fremitus, hyperresonance on percussion (due to the presence of air), and decreased breath sounds because the lung is unable to expand properly.

Incorrect answers:
A. A dull percussion sound is more typical of conditions like pleural effusion, not pneumothorax, which causes hyperresonance due to air in the pleural space.

C. The presence of dull percussion and decreased breath sounds does not align with pneumothorax findings.

D. Bronchial breath sounds are usually heard in areas of consolidation, not over a pneumothorax.

E. Increased fremitus and dull percussion are not expected with a pneumothorax.

Question 89:

The correct answer is D. Polychromasia. This healthy 22-year-old man lost approximately 20% of his total blood volume (acute hemorrhagic shock) from a motor vehicle collision. Following immediate surgical stabilization, his kidneys sense hypoxia and upscale the synthesis of erythropoietin (EPO) to stimulate massive bone marrow erythroid hyperplasia. In 10 days, this robust response manifests on a peripheral blood smear as polychromasia—the presence of large, immature red blood cells (reticulocytes) that exhibit a blue-gray tint due to residual ribosomal RNA necessary for active hemoglobin synthesis.

Incorrect answers:
A & E. Heinz bodies (oxidized, precipitated hemoglobin aggregates) and siderotic granules ( iron inclusions within siderocytes) are pathological structures classically seen in G6PD deficiency or lead poisoning/sideroblastic anemias, respectively, rather than a normal physiological response to hemorrhage.

B. Hypochromia refers to RBCs with an expanded central pallor due to deficient hemoglobin synthesis, typically requiring weeks to months of chronic iron deficiency or genetic thalassemia rather than acute blood loss.

C. Microspherocytosis describes small, dense erythrocytes lacking central pallor, a classic presentation of hereditary spherocytosis or autoimmune hemolytic anemia rather than healthy bone marrow regeneration.

Question 90:

The correct answer is B. This patient has manifestations suggestive of exocrine pancreatic insufficiency, including steatorrhea (fatty stools), significant weight loss, and a dry, scaly rash. These symptoms are consistent with fat malabsorption and subsequent deficiency of fat-soluble vitamins, such as vitamin A. Pancreatic insufficiency leads to decreased secretion of digestive enzymes (e.g., lipase), impairing the absorption of fats and fat-soluble vitamins. Vitamin A deficiency commonly manifests as xerosis (dry skin). Stool testing is a good test to screen for malabsorption due to pancreatitis.

Incorrect answers:
A. Magnesium deficiency typically presents with symptoms such as muscle cramps, fatigue, and, in severe cases, arrhythmias and neuromuscular excitability.

C. Vitamin B12 deficiency results in megaloblastic anemia and neurological symptoms, including loss of proprioception, ataxia, and weakness.

D. Vitamin C is water-soluble, and its deficiency causes scurvy, characterized by bleeding gums, perifollicular hemorrhages, and impaired wound healing.

E. Zinc deficiency can cause dermatitis, especially around body orifices (mouth, anus), impaired wound healing, anosmia, and hypogonadism in men. The dry scaly rash in this patient is more consistent with vitamin A deficiency rather than zinc deficiency.

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Question 91:

The correct answer is C. Oxygen Tension: Decreased, Osmolality: Increased. This 22-year-old woman with sickle cell disease presents with gross, painless hematuria and a CT scan showing renal papillary necrosis. The renal medulla, specifically the papillae, is highly prone to microvascular occlusion and infarction (sickling) in patients with sickle cell trait or disease. The extreme physiological environment of the renal medulla features a decreased oxygen tension (due to a countercurrent exchange system that leaves the tissue relatively hypoxic) and a remarkably increased osmolality (due to the concentration of solutes by the countercurrent multiplier mechanism).

These two physiological conditions directly promote the polymerization of deoxygenated hemoglobin S (HbS). The resulting erythrocyte sickling occludes the vasa recta, inducing microvascular ischemia, localized infarction, and ischemic necrosis of the renal papillae.

Incorrect answers:
A & B. An increased oxygen tension prevents HbS polymerization by keeping hemoglobin in its relaxed, oxygenated state, which protects against red blood cell deformability and sickling.

D. A decreased osmolality (hypotonic environment) keeps red blood cells relatively hydrated, which lowers the intracellular concentration of HbS and decreases the likelihood of polymer nucleation and sickling.

Question 92:

The correct answer is C. Leukotriene. This 34-year-old woman presents with classic signs of acute appendicitis, including severe right lower quadrant abdominal pain with rebound tenderness, high fever (40.0C), and tachycardia (121/min). Following an appendectomy, the photomicrograph of the resected tissue demonstrates a dense transmural inflammatory infiltrate within the muscularis propria composed predominantly of neutrophils (characterized by multi-lobed nuclei), accompanied by tissue congestion and hemorrhage.

The recruitment and migration of neutrophils into the inflamed tissue are driven by specific chemotactic factors. Among the given choices, leukotrienes function as powerful leukocyte chemotactic agents, stimulating neutrophil recruitment and activation at the site of acute inflammation. Other key high-yield neutrophil chemoattractants include IL-8, C5a, and bacterial formyl-methionyl peptides (f-Met).

Incorrect answers:
A & B. Bradykinin and histamine are vasoactive mediators responsible for driving tissue vasodilation and increasing vascular permeability, as well as mediating pain sensitization, rather than acting as primary neutrophil chemoattractants.

D. Lipoxins serve as anti-inflammatory regulators that inhibit neutrophil chemotaxis, helping to promote the resolution phase of acute inflammation.

E. Thromboxane is synthesized via the COX pathway and acts primarily as a potent vasoconstrictor and inducer of platelet aggregation, playing no direct role in leukocyte chemotaxis.

Question 93:

The correct answer is B. Brief psychotic disorder. This 15-year-old boy presents with an acute onset of psychotic symptoms, including auditory hallucinations (“the voice tells me so”), persecutory delusions (“dark eyes… are warning me”), and disorganized speech. His symptoms began shortly after a psychological stressor (hearing about a school shooting) and have lasted for less than 1 month. A negative urine toxicology screen rules out substance-induced psychosis. Because the duration of active symptoms is > 1 day but < 1 month with an eventual return to normal functioning, it meets the criteria for brief psychotic disorder.

Incorrect answers:
A. Adjustment disorder with anxiety involves emotional or behavioral symptoms in response to a stressor, but cannot include core features of psychosis like delusions or hallucinations.

C. Post-traumatic stress disorder (PTSD) causes hyperarousal, avoidance, and re-experiencing, but does not present with an acute onset of disorganized speech and active auditory hallucinations.

D. Schizophrenia requires a continuous duration of illness lasting for 6 months, whereas this patient’s presentation has only lasted a week.

E. Schizotypal personality disorder is a chronic, lifelong pattern of social deficits and eccentric behavior that does not manifest as an acute, sudden break into active hallucinations and disorganized speech.

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Question 94:

The correct answer is B. This boy has a history of fever, headache, photophobia, and nuchal rigidity, which are classic signs of meningitis. A normal CSF glucose level makes bacterial meningitis unlikely, as bacterial infections typically cause low CSF glucose levels. Furthermore, the presence of elevated lymphocytes suggests viral (aseptic) meningitis. Given his age and history of camping, enterovirus is the most probable cause.

Bacterial meningitis is marked by elevated opening pressure and cerebrospinal fluid (CSF) findings showing increased white blood cells (predominantly neutrophils), elevated protein levels, and reduced glucose.

Viral meningitis typically presents with a normal opening pressure and cerebrospinal fluid (CSF) findings showing increased lymphocytes, with either normal or slightly elevated protein levels and normal glucose concentrations.

Fungal meningitis typically presents with opening pressure that is normal to elevated, increased protein levels, and glucose levels that are normal to low.

Incorrect answers:
A. Adenovirus usually presents with respiratory symptoms, which are not evident in this patient, making adenovirus an unlikely cause.

C. Herpes simplex virus (HSV) meningitis is typically linked to a history of genital herpes, which this patient lacks, making the diagnosis less probable.

D & E. Bacterial meningitis (e.g., Streptococcus pneumoniae, Neisseria meningitidis) – Bacterial meningitis generally presents with low CSF glucose levels and a predominance of neutrophils, whereas this patient exhibits normal glucose levels and a predominance of lymphocytes, thus ruling out bacterial causes.

Question 95:

The correct answer is B. Decreased respiratory muscle strength. This 85-year-old woman presents with progressive shortness of breath, a cachectic state (BMI = 15 kg/m2), and a markedly reduced diaphragmatic excursion (1.5 cm). Her pulmonary function tests (PFTs) and flow-volume loop demonstrate a classic restrictive pattern:

Significantly decreased lung volumes (TLC = 54%, FVC = 26%).
A normal-to-increased ratio (FEV1/FVC = 1.0).
A rightward-shifted, narrowed flow-volume loop matching low lung volumes.

Crucially, her diffusing capacity for carbon monoxide is completely intact (DLCO = 100%), ruling out intrinsic lung parenchymal diseases (like pulmonary fibrosis). Combined with her severe cachexia and poor diaphragmatic excursion, the restrictive defect is caused by extrinsic, extrapulmonary respiratory pump failure due to decreased respiratory muscle strength (such as respiratory muscle weakness or amyotrophic lateral sclerosis/sarcopenia).

Incorrect answers:
A, C & D. Decreased lung elasticity (fibrosis), decreased surfactant production, and increased alveolar surface tension characterize intrinsic restrictive lung diseases. These pathologies cause architectural damage to the alveolar-capillary membrane, which would characteristically reduce the DLCO (less than 80%).

E. Increased bronchial mucous secretions causes obstructive lung disease (such as chronic bronchitis), which increases lung volumes (TLC, FRV, RV) and produces a leftward-shifted, scooped flow-volume loop with a decreased FEV1/FVC ratio (less than 0.70).

F. Increased lung compliance is seen in emphysema due to the destruction of alveolar elastic fibers, leading to air trapping, hyperinflation (increased TLC and RV), and a classic obstructive profile.

Question 96:

The correct answer is C. This patient has a long history of menstrual pain that typically starts on the first day of her period, indicating primary dysmenorrhea. Dysmenorrhea is mainly caused by excessive prostaglandin production, which leads to uterine contractions and pain in young females. The first-line treatment for dysmenorrhea in an otherwise healthy female is nonsteroidal anti-inflammatory drugs (NSAIDs), like ibuprofen. NSAIDs function by inhibiting prostaglandin synthesis, which helps reduce uterine contractions and relieve pain.

Incorrect answers:
A. Drugs that inhibit estrogen synthesis are used to treat certain types of cancers, such as breast cancer, but they are not indicated for dysmenorrhea.

B. A decrease in 11β-hydroxylase activity results in increased androgen production in the adrenal gland and is treated with specific medications for congenital adrenal hyperplasia (CAH) or endocrine disorders such as Cushing syndrome, but plays no role in treating dysmenorrhea.

D & E. FSH and LH stimulation is used to induce ovulation, often within infertility treatment, but these hormones do not play a role in managing primary dysmenorrhea.

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Question 97:

The correct answer is D. Increased aquaporin expression. This 60-year-old woman collapsed from dehydration after running a marathon. Physical examination shows volume depletion, including low blood pressure (90/60 mm Hg), tachycardia (110/min), dry mucous membranes, and decreased skin turgor.

In response to severe intravascular volume depletion and high plasma osmolality, the posterior pituitary gland releases antidiuretic hormone (ADH / vasopressin). ADH binds to V2 receptors on the basolateral membrane of principal cells in the renal collecting ducts. This activates a signaling cascade leading to the insertion of aquaporin-2 water channels into the apical (luminal) membrane. This increased aquaporin expression allows for massive water reabsorption back into the medullary interstitium, conserving water and producing highly concentrated urine.

Incorrect answers:
A. Decreased ADH receptors causes nephrogenic diabetes insipidus, preventing water reabsorption and resulting in massive, dilute polyuria that worsens dehydration.

B. Decreased renin production is incorrect because severe volume depletion activates the renin-angiotensin-aldosterone system, increasing renin production to help maintain blood pressure.

C. Decreased urine osmolality is incorrect because the activation of ADH causes maximum water retention, resulting in an increased urine osmolality (highly concentrated urine).

E. Increased glomerular filtration rate (GFR) is incorrect because severe dehydration causes sympathetic activation and renal afferent arteriolar vasoconstriction, which decreases the GFR.

F. Increased reabsorption of albumin is incorrect because albumin is restricted from filtration by the glomerular basement membrane; it is not filtered or reabsorbed by the tubular system during volume depletion.

Question 98:

The correct answer is D. This 19-year-old college freshman presents with a 6-day history of fever, malaise, and swelling of the parotid glands, which are hallmark features of mumps. Mumps is caused by the mumps virus, a member of the paramyxovirus family.

Mumps is a viral illness that typically causes parotitis, or inflammation of the parotid glands, and systemic symptoms like fever and malaise. It primarily affects unvaccinated individuals or those with waning immunity. Key clinical features include:

Parotid gland swelling: Often bilateral, causing tenderness and difficulty with chewing or speaking.
Fever and malaise: Non-specific viral symptoms.
Other Complications (in some cases):
a. Orchitis: Testicular inflammation, particularly in post-pubertal males.
b. Aseptic meningitis: Mild meningeal irritation.
c. Pancreatitis: Rare but possible

Incorrect answers:
A. Epstein-Barr virus (EBV) causes infectious mononucleosis, which may present with fever, pharyngitis, and posterior cervical lymphadenopathy but does not typically cause parotid gland swelling.

B. Hepatitis B virus causes liver inflammation, not parotitis. It may present with jaundice, fatigue, and right upper quadrant pain, which are not seen here.

C. Measles is characterized by fever, cough, coryza, conjunctivitis, and a maculopapular rash. It does not cause isolated parotid gland swelling.

E. Rubella causes mild fever, a maculopapular rash, and posterior auricular and occipital lymphadenopathy but does not lead to parotitis.

Question 99:

The correct answer is D. CTLA4. This 64-year-old man has unresectable, metastatic melanoma. Malignant melanoma cells often exploit immune checkpoint pathways to evade T-cell-mediated destruction. Under normal conditions, T-cell activation requires two signals: binding of the T-cell receptor (TCR) to the antigen-MHC complex, and a co-stimulatory signal (B7 on antigen-presenting cells binding to CD28 on T-cells).

CTLA-4 is an inhibitory receptor expressed on T-cells that competes with CD28 to bind B7 with much higher affinity. When CTLA-4 binds B7, it delivers an inhibitory signal that downregulates T-cell activation, allowing the tumor to escape immune surveillance. Therapeutic antibody blockade targeting CTLA-4 (such as ipilimumab) prevents this inhibitory interaction. This restores costimulatory signaling, reinstates T-cell proliferation, and enhances the anti-tumor immune response to destroy the metastatic melanoma cells.

Incorrect answers:
A & B. Targeting B7 or CD28 with blocking antibodies would prevent essential co-stimulatory signaling, which inhibits T-cell activation and worsens the immune system’s ability to fight the tumor.

C. CD40 is a costimulatory protein on antigen-presenting cells that interacts with CD40L on T-cells to promote macrophage and B-cell activation; blocking it would impair cell-mediated immunity.

E. FcgR (Fc-gamma receptor) binds the constant Fc region of IgG antibodies to mediate phagocytosis and antibody-dependent cellular cytotoxicity; blocking it does not provide targeted checkpoint inhibition for T-cell reactivation against melanoma.

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Question 100:

The correct answer is D. An interpreter is necessary when a patient does not understand or has difficulty understanding the physician’s language. A professional interpreter, whether in person or via telephone, is always preferred to ensure precise communication. Relatives should not serve as interpreters, as they may have trouble with medical terminology or introduce bias due to their relationship with the patient.

Incorrect answers:
A. A relative, such as a spouse, should never be used as an interpreter until the patient himself wants to. It should be recorded in a chart if the patient desires to use a family member as interpreter.

B. A telephone interpreter service should be used immediately—consulting a hospital committee is unnecessary.

C. If a hospital interpreter is unavailable, a telephone interpreter should be used without delay.

Block 6

Question 101:

The correct answer is C. The patient’s symptoms, including coughing spells lasting 1-2 minutes, often followed by vomiting, are characteristic of whooping cough (pertussis). The lack of vaccinations supports this diagnosis, as pertussis is most commonly seen in unvaccinated individuals.

A gram-negative coccobacillus causes pertussis and has three clinical stages. The catarrhal stage is the first one, lasting up to 2 weeks with a mild cough. The paroxysmal stage lasts 2 to 6 weeks and is characterized by severe coughing spells, a whooping sound, and vomiting after a cough spell. The third stage, the convalescent stage, is when the cough gradually resolves over weeks to months.

The drug of choice for treating Bordetella pertussis is macrolides (e.g., azithromycin and erythromycin), which effectively reduce symptoms’ severity and prevent transmission.

Incorrect answers:
A. Cephalosporins work by inhibiting cell wall synthesis and have a broad spectrum, including Gram-positive cocci and some Gram-negative infections such as Pseudomonas. Still, they are not the drug of choice for pertussis.

B. Fluoroquinolones work by inhibiting bacterial topoisomerases, and their spectrum includes Gram-negative rods such as Pseudomonas and some Gram-positive organisms, but they are not the drug of choice for pertussis.

D. Penicillins are also cell wall inhibitors with activity against Gram-positive organisms and Gram-negative cocci, but they are usually not the drug of choice because of resistance

E. Sulfonamides inhibit folate synthesis and are alternative drugs for pertussis if macrolides are contraindicated.

Question 102:

The correct answer is F. This 25-year-old man with a known history of amphetamine use presents with symptoms of sympathomimetic toxicity, including restlessness, tremor, palpitations, tachycardia, and hypertension. These symptoms are caused by increased release of biogenic amines (e.g., norepinephrine) at synaptic nerve terminals, which is the primary mechanism of action of amphetamines.

Amphetamines are stimulants that increase synaptic concentrations of norepinephrine by two mechanisms:

1. Increased release of biogenic amines:

Inside the presynaptic neuron, amphetamines inhibit the vesicular monoamine transporter (VMAT), causing accumulation of monoamines in the cytosol.
This leads to reverse transport of monoamines into the synaptic cleft, increasing their extracellular levels.

2. Inhibition of reuptake:

Amphetamines inhibit reuptake of norepinephrine and dopamine into presynaptic neurons, further enhancing their concentrations in the synaptic cleft.

Incorrect answers:
A. The primary mechanism of amphetamine toxicity is increased release of monoamines, not decreased intracellular metabolism.

B. MAO inhibition is not the main mechanism of amphetamine action.

C & E. The main mechanism involves postsynaptic receptor activation due to increased monoamine levels, not presynaptic receptor activation.

D. Amphetamines do not increase monoamine metabolism; they inhibit reuptake and enhance release.

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Question 103:

The correct answer is E. This patient most likely has a stretched aortic annulus. The clinical presentation of severe, sharp upper back pain, along with hemodynamic instability (hypotension and tachycardia), is characteristic of aortic dissection. The presence of a diastolic murmur over the left sternal border suggests aortic valve regurgitation, which can occur due to dissection of the aortic root. When the dissection extends into the aortic annulus, it causes dilatation of the aortic root, which leads to incompetence of the aortic valve. This results in aortic regurgitation. The patient’s tall stature is a key feature of Marfan syndrome, which is often associated with aortic root dilation and increased risk for aortic dissection.

Incorrect answers:
A. Atrial septal defect typically presents with a wide, fixed splitting of the second heart sound and is not associated with acute hemodynamic collapse or diastolic murmurs.

B. Mitral stenosis is characterized by an opening snap and a diastolic murmur at the apex, which does not match this patient’s murmur location or presentation.

C. Papillary muscle rupture is a complication of myocardial infarction and leads to acute mitral regurgitation, causing a holosystolic murmur at the apex rather than a diastolic murmur.

D. Perforated tricuspid valve is typically associated with infective endocarditis, particularly in intravenous drug users, and presents with a systolic murmur rather than a diastolic murmur.

Question 104:

The correct answer is E. Myasthenic (Lambert-Eaton) syndrome. This 78-year-old woman with bronchogenic carcinoma (likely small cell lung cancer) presents with pre-operative proximal muscle weakness and prolonged paralysis after neuromuscular blockade. The key to this question lies in evaluating the electrophysiologic data from the intercostal muscle biopsy:

Miniature end-plate potential (MEPP) amplitude is normal (1 mV): This shows that individual vesicles contain a normal amount of acetylcholine (ACh) and that the postsynaptic ACh receptors are fully functional.
Response to exogenous Acetylcholine is normal (1 mV): This confirms that the postsynaptic membrane responds completely normally when ACh is applied directly, ruling out postsynaptic receptor destruction.
End-plate potential (EPP) amplitude is markedly reduced (3 mV vs 10 mV in healthy muscle): The EPP is generated by the simultaneous release of multiple ACh vesicles upon nerve stimulation. A low EPP with a normal MEPP proves that the underlying defect is a decrease in the number of ACh vesicles released per nerve impulse (a presynaptic defect).

This pattern is diagnostic of Lambert-Eaton myasthenic syndrome (LEMS), a paraneoplastic condition caused by autoantibodies against presynaptic voltage-gated calcium channels (VGCC). This blocks calcium influx during nerve depolarization, severely reducing the calcium-dependent exocytosis of ACh vesicles.

Incorrect answers:
A & B. Adverse effects of pancuronium (a competitive antagonist) or sevoflurane would act postsynaptically or generally, but this patient had severe weakness before the operation, meaning the drug effects are secondary to her underlying disease.

C. Cholinergic crisis is caused by excessive ACh stimulation (such as organophosphate poisoning), which would cause depolarizing blockade and widespread systemic autonomic overactivity.

D. Myasthenia gravis is a postsynaptic disorder caused by autoantibodies against nicotinic ACh receptors. Destruction of these receptors characteristically causes a decreased MEPP amplitude and a decreased response to exogenous acetylcholine, which directly contradicts this patient’s findings.

Question 105:

The correct answer is C. Calcium-sensing receptor. This asymptomatic 28-year-old man presents with mild hypercalcemia (Serum Calcium = 11.3 mg/dL), borderline high parathyroid hormone (PTH = 112 pg/mL), and an extremely low urinary calcium excretion (Fractional excretion of calcium is less than 1%). His family history is also positive for abnormal serum calcium levels. This classic clinical triad in an asymptomatic patient is highly diagnostic of Familial Hypocalciuric Hypercalcemia (FHH).

FHH is an autosomal dominant disorder caused by an inactivating mutation in the G-protein-coupled calcium-sensing receptors (CaSR) found in the parathyroid glands and the kidneys.

In the parathyroid gland: The defective receptors require much higher extracellular calcium concentrations to suppress PTH secretion, shifting the set-point for calcium homeostasis upward and leading to inappropriate PTH release despite hypercalcemia.
In the kidneys: The defective receptors lead to increased renal tubular reabsorption of calcium, resulting in profound hypocalciuria (low urine calcium).

Because the condition is benign and asymptomatic, it requires no treatment, making it critical to distinguish from primary hyperparathyroidism to avoid unnecessary parathyroidectomy.

Incorrect answers:
A & B. Mutations in calcitonin or calcitonin gene-related peptide receptors are not associated with familial syndromes of combined hypercalcemia and hypocalciuria.

D. Inactivating mutations of the parathyroid hormone receptor would cause resistance to PTH, presenting as hypocalcemia and hyperphosphatemia (pseudohypoparathyroidism), rather than hypercalcemia.

E. Receptor activator of nuclear factor kB (RANK) is involved in osteoclast differentiation and bone resorption; mutations here cause skeletal disorders like osteopetrosis rather than isolated FHH.

F. Vitamin D receptor mutations cause hereditary vitamin D-resistant rickets, presenting with severe hypocalcemia, rickets, and elevated vitamin D levels, completely opposite to this patient’s findings.

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Question 106:

The correct answer is C. “I’d like to hear about your goals for the time you have remaining.” This 32-year-old woman has terminal, stage IV breast cancer complicated by recurrent malignant pleural effusions. After undergoing multiple uncomfortable large-volume thoracenteses and chemotherapy regimens without improvement, she states she is “done” with being in the hospital and refuses further invasive procedures.

When a terminally ill patient with decision-making capacity decides to discontinue disease-directed or invasive interventions, the most appropriate physician response is to respect patient autonomy, validate her feelings, and transition the focus of care from prolongation of life to comfort and quality of life. Asking about her goals allows the physician to explore her values, understand her wishes, and establish a comprehensive palliative or hospice care plan tailored to her preferences.

Incorrect answers:
A. This response is judgmental and inappropriately frames the patient’s valid decision to choose comfort care over invasive procedures as “giving up.”

B. This response uses guilt to pressure the patient into continuing interventions she explicitly refused, violating the core principle of patient autonomy.

D. While evaluating for clinical depression is reasonable in patients expressing a desire to stop treatment, refusing painful, non-curative interventions in stage IV cancer is a normal and rational decision. Delaying the discussion to force a psychiatric evaluation ignores her immediate care goals.

E. This response normalizes her dislike of the procedure but fails to address her statement about being “done” with hospitalizations, missing the opportunity to discuss her end-of-life goals.

Question 107:

The correct answer is C. This newborn has a 46XX karyotype (female) but displays ambiguous genitalia, featuring a structure that resembles a penis. This indicates virilization, which occurs due to excess androgen exposure during fetal development. The most common causes include congenital adrenal hyperplasia (CAH)—specifically, 21-hydroxylase or 11β-hydroxylase deficiency—or maternal androgen exposure.

Incorrect answers:
A. While congenital adrenal hyperplasia (CAH) causing virilization in females is typically associated with elevated 17-hydroxyprogesterone (17-OHP), deficient 17-hydroxyprogesterone can lead to androgen deficiency in CAH, causing ambiguous genitalia in males (46XY), not females.

D. The sex-determining region Y (SRY) gene is located on the Y chromosome, making its presence highly unlikely in a 46XX female.

E. 5α-reductase deficiency impacts DHT production, resulting in undescended testes and ambiguous genitalia in 46XY males, rather than causing virilization in females.

Question 108:

The correct answer is D. This patient works in wet shoes and socks, creating a warm, moist environment that promotes fungal growth. The infection’s location (interdigital spaces) and itching and burning sensation strongly suggest tinea pedis (a dermatophytosis).

Antifungals used for dermatophytoses include terbinafine (a squalene epoxidase inhibitor), griseofulvin (a microtubule inhibitor), and sometimes azoles (ergosterol synthesis inhibitors). Only squalene monooxygenase (squalene epoxidase) is listed as an answer choice. Additionally, terbinafine is the preferred drug for tinea pedis, making it the correct answer.

Incorrect answers:
A. No antifungal effective against dermatophytes works by inhibiting chitin synthase.

B. 1,3-β-D-Glucan synthase is inhibited by echinocandins, which are used for systemic fungal infections, not dermatophytosis.

C. Phospholipase D is not a known target of any commonly used antifungal drug.

E. Flucytosine indirectly inhibits Thymidylate synthase, which is used for systemic mycoses. Thus, it is inappropriate for dermatophytosis.

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Question 109:

The correct answer is B. This patient likely has damage to the left medial longitudinal fasciculus (MLF), which connects the oculomotor and abducens nerves to coordinate horizontal eye movements. The MLF transmits signals from the abducens nucleus to the contralateral oculomotor nucleus. Damage to the left MLF prevents the left oculomotor nerve from synchronizing with the right abducens nerve, resulting in impaired eye movement, especially during rightward gaze. This causes the left eye to fail to adduct, a hallmark of internuclear ophthalmoplegia (INO), which is often associated with multiple sclerosis (MS).

Incorrect answers:
A. A left abducens nerve (CN VI) lesion would cause impaired abduction of the left eye.

C. A lesion of the left abducens nucleus would result in ipsilateral horizontal gaze palsy due to affection of the left abducens nerve and the right MLF connecting the left abducens and right oculomotor nerve.

D. A right abducens nerve (CN VI) lesion would cause impaired abduction of the right eye.

E. A lesion of the right MLF would cause impaired adduction of the right eye on leftward gaze, which does not match this patient’s findings.

F. A lesion of the right abducens nucleus would result in ipsilateral horizontal gaze palsy due to affection of the right abducens nerve and the left MLF connecting the right abducens and left oculomotor nerve.

Question 110:

The correct answer is A. This patient has poorly controlled hypertension, evidenced by the high blood pressure of 180/105, headache, and shortness of breath. Ophthalmoscopic examination typically shows arteriovenous nicking due to the compression of the artery when it crosses over the vein, which is a classic sign of severe uncontrolled hypertension. Other findings include flame-shaped hemorrhages, and cotton-wool spots.

Incorrect answers:
B. Melanocytes in the uvea are unlikely to be associated with a history of hypertension. These are more commonly associated with abundant skin moles or melanocytic lesions and not hypertension.

C. Optic neuritis typically presents with visual changes and pain, often seen in conditions like multiple sclerosis, rather than hypertension.

D. Posterior subcapsular cataracts are associated with gradual vision loss, glare, and halos around light but are not linked with high blood pressure.

E. Tractional retinal detachment is usually attributed to diabetic retinopathy and presents with gradual vision loss, which is not characteristic of the hypertensive changes seen in this patient.

Question 111:

The correct answer is D. Obstruction of bile flow. This 16-day-old male newborn presents with jaundice (conjunctival icterus), dark urine staining his diapers, and a markedly elevated total bilirubin (20 mg/dL) with a predominant direct (conjugated) bilirubin component (14 mg/dL). Conjugated hyperbilirubinemia in a newborn is always pathologic and indicates cholestasis.

The presence of dark urine proves that the kidneys are excreting water-soluble conjugated bilirubin, which only happens when there is an obstruction of bile flow (such as biliary atresia or a choledochal cyst) preventing bile from draining into the duodenum. This physical block leads to a backup of conjugated bilirubin into the systemic circulation, causing elevated direct bilirubin and mildly elevated liver enzymes (AST, ALT, GGT) due to secondary localized hepatic inflammation.

Incorrect answers:
A. Deficiency of UDP-glucuronosyltransferase is seen in Crigler-Najjar or Gilbert syndromes. This deficiency impairs the conjugation process, resulting in isolated indirect (unconjugated) hyperbilirubinemia, meaning the direct bilirubin would be normal.

B. Hemolysis increases the breakdown of hemoglobin, overloading the liver with unconjugated bilirubin. This produces indirect hyperbilirubinemia, not direct, and would typically present with a drop in hemoglobin/hematocrit levels.

C. Low activity of the bile canalicular MRP transporter describes Dubin-Johnson syndrome. While it does cause conjugated hyperbilirubinemia, it is a benign, inherited condition that presents later in life with a black liver, not as an acute, progressive neonatal cholestatic presentation with significant enzyme elevation.

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Question 112:

The correct answer is B. Fibromuscular dysplasia. This 21-year-old woman presents with severe asymptomatic hypertension (220/100 mm Hg), a faint abdominal bruit, and signs of chronic pressure overload leading to left ventricular hypertrophy on ECG. In a young, otherwise healthy female patient, the sudden discovery of severe secondary hypertension accompanied by an abdominal bruit is highly characteristic of fibromuscular dysplasia (FMD).

FMD is a non-atherosclerotic, non-inflammatory vascular disease that causes abnormal tissue growth within the arterial walls, most commonly affecting the renal and internal carotid arteries. When it involves the renal arteries, it typically leads to stenosis (often showing a classic string-of-beads appearance on angiography). The resulting stenosis reduces renal blood flow, triggering a massive activation of the renin-angiotensin-aldosterone system (RAAS), which drives the development of severe renovascular hypertension.

Incorrect answers:
A. Extrinsic arterial compression (such as by a tumor) can cause renal ischemia and hypertension, but it is rare and not the primary suspected etiology in a young woman with this clinical profile.

C. Mid-aortic syndrome is a rare condition characterized by segmental narrowing of the abdominal aorta, which can cause hypertension and abdominal bruits, but it typically presents during childhood or early adolescence with claudication and blood pressure discrepancies between upper and lower extremities.

D. Premature atherosclerosis is an exceedingly rare cause of renal artery stenosis in a 21-year-old woman without severe underlying metabolic or genetic risks (such as homozygous familial hypercholesterolemia), as atherosclerosis predominantly affects older males with established cardiovascular risk factors.

E. Spontaneous renal artery dissection is an acute, dramatic event that presents with sudden, severe flank or abdominal pain and hematuria, rather than an asymptomatic incidental finding during a routine physical examination.

Question 113:

The correct answer is D. In a normal distribution, 68% of values fall within ±1 SD, 95% within ±2 SD, and 99.7% within ±3 SD of the mean. The mean leukocyte count is 7500, and the distribution is normal, meaning that 50% of values are above and 50% are below the mean. Make sure you remember The 68, 95 and 99.7 Rule.

Incorrect answers:
A. This represents the values that fall between -2 SD and +2 SD, which includes 95% of the population.

B. This represents values that are lower than -1 SD (16%) and higher than +1 SD (16%), Sum values are 32%.

C. This represents the values that fall between -1 SD and +1 SD, which includes approximately 68% of observations.

E. This represents the values above +2 SD, which account for only 2.5% of the population.

Question 114:

The correct answer is C. This 10-year-old boy presents with mucocutaneous bleeding (nosebleeds, gingival bleeding, easy bruisability) and a normal platelet count. Laboratory findings indicate normal platelet adhesion to ristocetin, but impaired platelet aggregation in response to thrombin. These findings are diagnostic of Glanzmann thrombasthenia, which is caused by a defect in glycoprotein IIb-IIIa (GpIIb-IIIa), a critical protein for platelet aggregation.

Glanzmann thrombasthenia is an autosomal recessive disorder caused by a deficiency or dysfunction of GpIIb-IIIa, an integrin on the platelet surface. This protein mediates the final common step of platelet aggregation by binding fibrinogen and forming cross-links between adjacent platelets. Without functional GpIIb-IIIa, platelets cannot aggregate properly, resulting in a bleeding disorder. Key features of Glanzmann thrombasthenia include:

Mucocutaneous bleeding: Epistaxis, gingival bleeding, and easy bruising are common manifestations.
Normal platelet count and morphology: Differentiates it from conditions like immune thrombocytopenic purpura (ITP) or bone marrow disorders.
Normal response to ristocetin: Ristocetin-induced platelet aggregation tests platelet adhesion via von Willebrand factor (vWF) and glycoprotein Ib (GpIb). Since these pathways are intact, the response to ristocetin is normal.
Impaired aggregation in response to other agonists: Platelets fail to aggregate in response to stimuli like ADP, epinephrine, or thrombin due to the defective GpIIb-IIIa complex.

Incorrect answers:
A. Deficiency in factor VII affects the extrinsic coagulation pathway and results in prolonged prothrombin time (PT). This patient has a normal PT and no signs of deep tissue bleeding, making this unlikely.

B. Fibrinogen deficiency or dysfunction (dysfibrinogenemia) would impair clot formation and prolong clotting times (PT and aPTT). This patient’s clotting times are normal, ruling this out.

D. Granule storage pool disorders affect platelet secretion, not aggregation. Platelet aggregation in response to agonists would still be normal in these disorders.

E. von Willebrand disease affects platelet adhesion and is associated with a prolonged bleeding time and impaired response to ristocetin. This patient has a normal response to ristocetin, ruling out vWF-related conditions.

Question 115:

The correct answer is C. Geographically isolated population (founder effect). Iceland was colonized by a small Norse/Celtic group; centuries of isolation with limited immigration caused low genetic diversity, so the founder-carried 999del5 allele became overrepresented.

Incorrect answers:
A. Bottleneck from breast cancer doesn’t apply, BRCA2 causes cancer after reproductive age, so no selection pressure

B. Genetic heterogeneity is the opposite of what’s described — one mutation dominates

D. No established heterozygote advantage for BRCA2 carriers

E. Random mating in a large population maintains Hardy-Weinberg equilibrium and would keep allele frequency low, Iceland violates these assumptions.

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Question 116:

The correct answer is B. This 65-year-old woman with episodes of syncope, a pedunculated intracardiac mass, and the histological findings in the provided image most likely has a cardiac myxoma, which is commonly located in the left atrium. The histological image provided shows stellate-shaped or spindle cells embedded in a myxoid stroma, which is characteristic of a cardiac myxoma. This confirms the diagnosis.

Cardiac myxomas are the most common primary cardiac tumors in adults, and about 80% of them occur in the left atrium. They can cause symptoms through:

Obstruction of blood flow: A pedunculated myxoma can intermittently obstruct the mitral valve, causing symptoms like syncope or dyspnea, as seen in this patient.
Systemic embolization: Fragments of the tumor may embolize, leading to strokes or other ischemic events.
Constitutional symptoms: Myxomas can secrete interleukin-6 (IL-6), causing fever, weight loss, or malaise.

Incorrect answers:
A. This location is not typical for cardiac myxomas. Tumors in this area are more likely to be rhabdomyomas, fibromas, or sarcomas.

C. Cardiac myxomas are rare in the ventricles, with most arising in the atria.

D. While myxomas can occur here, the left atrium is the most common site.

E. Myxomas are extremely rare in the ventricles and would not fit this clinical presentation.

Question 117:

The correct answer is D. Preformed antibodies in the recipient. This 45-year-old woman experiences immediate failure of her transplanted kidney during surgery. Within 10 minutes of restoring blood flow (anastomosis), the graft becomes swollen and shows multiple areas of hemorrhage. This intraoperative presentation is a classic description of hyperacute transplant rejection.

Hyperacute rejection occurs within minutes to hours after vascular anastomosis. It is mediated by preformed, circulating antibodies in the recipient against donor antigens (such as anti-ABO blood group or anti-HLA antibodies). When the donor organ is connected to the recipient’s blood supply, these preformed antibodies immediately bind to the vascular endothelial cells of the graft. This activates the complement cascade, causing severe endothelial damage, extensive thrombosis, fibrin deposition, and ischemic necrosis, leading to a swollen, mottled, and non-functional transplant.

Incorrect answers:
A & B. Perfusate hypersensitivity refers to a reaction against solutions used to preserve the organ, which causes systemic allergic features or anaphylaxis rather than localized, immediate intraoperative graft thrombosis and hemorrhage.

C. Preformed antibodies in the donor is incorrect because rejection is driven by the recipient’s immune system identifying and attacking the foreign donor tissue, not the other way around.

E & F. Sensitized CD8 T lymphocytes mediate acute cellular rejection, which is a type IV hypersensitivity reaction. This process requires time for T-cell proliferation and infiltration, typically manifesting days to weeks (not minutes) after transplantation.

Question 118:

The correct answer is A. Bone remodeling. This 65-year-old man with a 2-year history of multiple myeloma presents with a sudden, non-traumatic midshaft femoral fracture (pathologic fracture). Multiple myeloma is a plasma cell dyscrasia characterized by the clonal proliferation of malignant plasma cells within the bone marrow.

The underlying mechanism of bone destruction in multiple myeloma involves a severe imbalance in normal bone remodeling. Malignant plasma cells produce cytokines—predominantly RANK ligand (RANKL) and Interleukin-6 (IL-6)—that stimulate osteoclast differentiation and activation. Concurrently, plasma cells produce factors like dickkopf-1 (DKK1) that inhibit Wnt signaling, severely suppressing osteoblast differentiation. This unchecked osteoclastic bone resorption combined with impaired osteoblastic bone formation leads to systemic osteopenia, punched-out lytic bone lesions, and mechanical weakness, predisposing the patient to pathologic fractures even with normal daily activities.

Incorrect answers:
B. Osteoblast generation is severely decreased and suppressed in multiple myeloma due to tumor-secreted inhibitors, which prevents normal bone repair and contributes to the lytic lesions.

C. While serum calcium concentration is often elevated (hypercalcemia) due to massive osteoclastic bone resorption, the hypercalcemia itself is a consequence of the bone destruction, not the primary cellular or structural abnormality causing the physical fracture.

D. Serum 25-hydroxycholecalciferol (Vitamin D) concentration is a marker of nutritional status and systemic calcium homeostasis; its alteration is not the driving mechanism behind localized myeloma-induced lytic bone disease.

E. Serum parathyroid hormone (PTH) concentration is typically suppressed in multiple myeloma as a feedback response to the hypercalcemia caused by local osteolytic factors, meaning it does not drive the bone destruction.

Question 119:

The correct answer is C. Inversion. This 45-year-old man presents with left ankle pain, swelling, and inability to bear weight after twisting his foot. Physical examination reveals edema and localized tenderness just distal to the lateral malleolus. This clinical presentation is classic for a lateral ankle sprain, which is overwhelmingly caused by an inversion injury of the plantar-flexed foot.

During sudden or forced inversion, the lateral collateral ligaments of the ankle are stretched or torn because they resist excessive inward rolling of the foot. The most commonly injured ligament in this scenario is the anterior talofibular ligament (ATFL), followed by the calcaneofibular ligament (CFL). The localized tenderness directly below and in front of the lateral malleolus confirms injury to these specific lateral stabilizing structures.

Incorrect answers:
A & D. Dorsiflexion and isolated plantar flexion are generic movements at the talocrural joint. While hyper-dorsiflexion can cause a syndesmotic (high) ankle sprain, it does not typically present with isolated tenderness confined to the area just distal to the lateral malleolus.

B. Eversion injuries stress the medial side of the ankle. This mechanism injures the strong, multi-part deltoid ligament or causes an avulsion fracture of the medial malleolus, presenting with medial ankle pain and tenderness rather than lateral side pathology.

Question 120:

The correct answer is E. Ventilation-perfusion mismatch. This 3-year-old boy presents with sudden severe coughing, gagging, and decreased breath sounds on the left side after eating candy, which is highly indicative of acute foreign body aspiration. His oxygen saturation is reduced at 93% on room air, and the chest X-ray reveals a classic complication: foreign body obstruction of the left main bronchus.

The obstruction physically blocks airflow to the left lung, creating a severe localized ventilation-perfusion (V/Q) mismatch. Specifically, this represents a low V/Q ratio or “shunt effect” where affected alveolar units have zero ventilation (V = 0) but continue to receive normal blood perfusion (Q). Deoxygenated mixed venous blood passing through the non-ventilated left lung cannot pick up oxygen, and when it mixes with well-oxygenated blood from the normal right lung, it lowers the overall systemic oxygen saturation, causing hypoxemia.

Incorrect answers:
A & D. Chest wall compliance relates to the elasticity and mechanical expandability of the thoracic cage (altered in conditions like kyphoscoliosis or obesity hypoventilation syndrome). It is not acutely altered or responsible for hypoxemia in a localized focal airway obstruction.

B. A diffusion defect occurs when there is architectural thickening or destruction of the alveolar-capillary membrane (such as in pulmonary fibrosis or acute respiratory distress syndrome), which is not the mechanism behind an acute mechanical bronchial plugging.

C. An extrapulmonary right-to-left shunt involves structural cardiovascular anomalies that bypass the lungs entirely (such as Tetralogy of Fallot or a patent ductus arteriosus with Eisenmenger syndrome), whereas this patient’s shunt effect is completely intrapulmonary.

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That brings us to the end of the new Free 120 NBME Step 1 sample test answers and explanations. We hope this breakdown helped you better understand the reasoning behind the correct answers and why the other options were incorrect. Hopefully, this will give you an edge in acing your USMLE Step 1 exam!

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