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USMLE Step 2 CK High-Yield Dermatology Topics
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Dermatology
High Yield USMLE Step 2 CK Notes
Welcome to the Dermatology High-Yield Notes for Step 2 CK.
These are the most commonly tested topics on the exam—ideal for quick review in your last 2 weeks before the exam, when you’re short on time, or when you’re just starting out and want to focus on what is high yield. Created and peer-reviewed by physicians who scored 260+ on Step 2 CK in a Q&A style to reinforce active learning.
How to use this file
- Read the question ➔ cover the answer ➔ self-quiz before revealing.
- Review the NOTES ➔ they include high-yield details you don’t want to miss.
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Table of Contents
Dermatology
1. A patient presents with a 1-week history of an itchy, non-painful rash on his arm. It began as a small red spot and has since tripled in size. The exam shows a round, scaly lesion with central clearing. What is the most appropriate topical pharmacotherapy?
➔ Topical antifungal (e.g., clotrimazole or terbinafine) for tinea corporis
- Note: Classic appearance is an annular plaque with scaly, advancing border and central clearing, caused by Trichophyton rubrum.
- Diagnosis is clinical. KOH prep shows segmented hyphae.
- Treated with topical antifungals → clotrimazole, terbinafine, or ketoconazole
CDC/ Dr. Lucille K. Georg (https://commons.wikimedia.org/wiki/File:Ringworm_on_the_arm,_or_tinea_corporis_due_to_Trichophyton_mentagrophytes_PHIL_2938_lores.jpg), „Ringworm on the arm, or tinea corporis due to Trichophyton mentagrophytes PHIL 2938 lores“, marked as public domain, more details on Wikimedia Commons: https://commons.wikimedia.org/wiki/Template:PD-US
2. A 22-year-old man presents with hypopigmented and hyperpigmented patches on his chest and back that do not tan. KOH prep reveals spaghetti-and-meatball appearance of yeast and hyphae. What is the most likely diagnosis?
➔ Pityriasis versicolor
- Note: Caused by Malassezia species present with Hypo/hyperpigmented scaling macules on the chest and the back that worsen with sun.
- KOH preparation shows spaghetti-and-meatballs (hyphae + spores).
- Treated with topical or oral antifungals as selenium sulfide.
Sarahrosenau on Flickr.com (https://commons.wikimedia.org/wiki/File:Tinea_versicolor1.jpg), „Tinea versicolor1“, https://creativecommons.org/licenses/by-sa/2.0/legalcode
3. A 7-year-old boy is brought to the clinic with a patchy area of hair loss on his scalp that has been growing, and the area is itchy. Exam shows hair loss with scales and black dots. What is the diagnosis?
➔ Tinea capitis
- Note: Dermatophyte infection of scalp → scaly, erythematous plaques, hair loss, black dots. KOH prep: branching, septate hyphae.
- Treatment: oral antifungals (griseofulvin, terbinafine).
- VS Trichotillomania → irregular patches of hair loss, hairs of varying length, no inflammation or scarring.
- VS Androgenic alopecia → gradual hair loss over the vertex and frontal scalp over the years, there might be a family history. no inflammation or scarring.
Tinea capitis
Gzzz (https://commons.wikimedia.org/wiki/File:Teigne_tondante_enfant.jpg), https://creativecommons.org/licenses/by-sa/4.0/legalcode
4. A 25-year-old woman presents with a 5-day history of fever and redness, pain, and swelling of the left thigh. The exam reveals diffuse erythema and tenderness of the thigh without fluctuance. What is the most appropriate next step in management?
➔ Empiric oral antibiotics for cellulitis (e.g., cephalexin)
- Note: Bacterial skin infection with Streptococcus pyogenes (non-purulent) and Staphylococcus aureus (purulent).
- Breaches in the skin increase infection risk; atopic dermatitis/ tinea pedis/ wounds and ulcers.
- It has erythema, warmth, tenderness, and induration. No fluctuance (not abscess).
- Treated with oral β-lactam antibiotics (e.g., cephalexin).
5. A 6-year-old boy presents with honey-colored crusted lesions around his mouth. What is the most appropriate treatment?
➔ Topical mupirocin (Localized non-bullous impetigo → Treat with topical mupirocin)
- Note: Differentiate from:
○ Bullous impetigo → Larger flaccid bullae, usually caused by Staph aureus, lacks crusting early → Treat with → Oral cephalexin or dicloxacillin
○ HSV → Painful grouped vesicles on erythematous base → Forms ulcers, not honey-crusted
Impetigo
Mohammad2018 (https://commons.wikimedia.org/wiki/File:Impetigo_hetpatiforms.jpg), https://creativecommons.org/licenses/by-sa/4.0/legalcode
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6. A 32-year-old man presents with a 3-day history of progressive left calf redness, swelling, and pain. The exam reveals a 10-cm erythematous, indurated, fluctuant area on the calf with mild tenderness. Ultrasound shows a well-defined fluid collection. What is the most appropriate next step in management?
➔ Incision and drainage (fluctuant + fluid collection on ultrasound → must perform I&D first for source control)
- Note: Most common cause → Staphylococcus aureus
- Classic features → Erythema, warmth, fluctuance, +/- drainage, fluid collection on imaging.
- Treated with Incision & drainage and oral antibiotics.
7. A patient presents with a severely itchy rash that began on the right hand and forearm and has now spread to the left hand and waistline. His roommate recently developed a similar rash. Exam shows scattered papules and excoriations on the hand and waistline. What is the most appropriate pharmacotherapy?
➔ Topical permethrin
- Note: Scabies (Sarcoptes scabiei), transmitted by Skin-to-skin contact (e.g., roommate exposure).
- Pruritic rash (especially at night), and burrows and papules on hands, wrists, waistline, and genitalia
- Permethrin cream for patient and close contacts simultaneously, and wash bedding/clothing in hot water & dry on high heat.
Gzzz (https://commons.wikimedia.org/wiki/File:Scabies_hand_and_fingers_2.jpg), https://creativecommons.org/licenses/by- sa/4.0/legalcode
8. An 18-year-old woman presents with a pruritic facial rash that started around the lips and spread to the cheeks over 2 days. She has a chronic, itchy rash on her extremities since childhood. She has a history of asthma. Physical exam reveals vesicular rash on the face and lichenified eczematous plaques on the cheeks, neck, and flexor surfaces. What is the most likely diagnosis?
➔ Eczema herpeticum (HSV superinfection of atopic dermatitis)
- Note: Atopy: Chronic pruritus + personal or family history of asthma or allergy.
- Trigger for suspicion: Sudden vesicular rash in atopic skin → think HSV infection.
- Eczema herpeticum: HSV-1 infection → painful vesicles, punched-out erosions
- Treatment: Oral acyclovir (IV in severe or pediatric cases)
9. A 67-year-old white man presents with concern about a lesion on the back of his neck. He states it’s been present for years but has noticeably grown over the past 6 months. Exam reveals a 3-cm irregularly pigmented lesion with asymmetry and variegated colors at the base of the skull. What is the most appropriate next step in management?
➔ Excisional biopsy
- Note: Suspicion: Asymmetric, Irregular borders, variegated color, recent growth → think melanoma
- Next step is Full-thickness excisional biopsy to assess depth “ most important factor”.
- Kaposi sarcoma presents as painful red or violaceous flat patches in HIV patients, caused by HHV8.
Melanoma
Unknown authorUnknown author (https://commons.wikimedia.org/wiki/File:Melanoma.jpg), „Melanoma“, marked as public domain, more details on Wikimedia Commons: https://commons.wikimedia.org/wiki/Template:PD-US
10. A 50-year-old man presents with a painless, non-itchy lesion on his face that has slowly enlarged over the past 6 months. Physical exam shows a pearly papule with rolled borders and telangiectasia on the face. What is the most likely diagnosis?
➔ Basal cell carcinoma
- Note: Classic appearance: Pearly or translucent papule with rolled borders, telangiectasia, and possible central ulceration. Often described as a “pimple that won’t heal”.
- Histology shows basaloid cell nests with peripheral palisading and retraction from the dermis
- Treated with Mohs micrographic surgery (best for facial lesions or high-risk areas), Excision with 5 mm–1 cm margins elsewhere.
Basal cell carcinoma
No machine-readable author provided. John Hendrix assumed (based on copyright claims). (https://commons.wikimedia.org/wiki/File:Basal_cell_carcinoma.jpg), „Basal cell carcinoma“, marked as public domain, more details on Wikimedia Commons: https://commons.wikimedia.org/wiki/Template:PD-self
- Squamous cell carcinoma presents as a scaly plaque or a non-healing ulcer. Histology shows keratin pearls.
Sqamous cell carcinoma
Jayabal Pandiaraja1 and A. Shalini (https://commons.wikimedia.org/wiki/File:Micrograph_of_squamous_cell_carcinoma_metastasis_to_the_skin.jpg), https://creativecommons.org/licenses/by/4.0/legalcode
11. A 76-year-old man presents with a 5-year history of progressive redness and scaling over bald areas of his scalp. He is a retired coach with significant sun exposure. Exam reveals ill-defined, rough, scaly lesions on the scalp and upper extremities. What is the most likely diagnosis?
➔ Actinic keratosis (UV-induced epidermal dysplasia with risk of squamous cell carcinoma)
- Note: Ill-defined, rough, scaly papules on Sun-exposed areas (scalp, face, ears, forearms). Precursor to squamous cell carcinoma
- Treatment:
○ Cryotherapy. (localized lesions)
○ Topical 5-FU or imiquimod. (multiple lesions)
○ Biopsy if the lesion is refractory or suspicious for SCC.
12. A 35-year-old moderately obese woman presents with a 4-year history of recurrent, painful abscesses in both axillae. The episodes are becoming more frequent and severe, now interfering with her work. She has well-controlled type 2 diabetes managed with oral agents. What is the most appropriate management?
➔ Surgical excision (for chronic, severe hidradenitis suppurativa)
- Note: Chronic inflammation of pilosebaceous glands presenting as recurrent axillary abscesses, nodules, sinuses, and scars. It commonly affects the obese, diabetics, and smokers.
- It increases the risk of squamous cell carcinoma, depression, and suicide.
- Mild cases are managed with topical clindamycin. Severe/refractory cases require TNF-α inhibitors or surgical excision
13. An old patient with a 1-year history of a slowly enlarging, painless, non-itchy lesion on his right inner thigh. Physical exam shows well-demarcated, raised, dark pigmented plaques with a “stuck-on” appearance. What factor most likely increased his risk for these lesions?
➔ Increasing age (the most important risk factor for seborrheic keratosis)
- Note: Benign, round, raised, brown/tan lesions with a “stuck-on” greasy appearance.
- Age is the strongest and most consistent association
- Treatment: Not required unless for cosmetic reasons → cryotherapy or excision
- Sudden eruption of multiple SK lesions may indicate Leser-Trélat sign → workup for GI malignancy.
Lmbuga (https://commons.wikimedia.org/wiki/File:Queratose_seborreica_1.jpg), „Queratose seborreica 1“, marked as public domain, more details on Wikimedia Commons: https://commons.wikimedia.org/wiki/Template:PD-self
14. A 22-year-old man presents with a 3-month history of thick, flaky fingernails and dry, scaly red patches on his forearms and chest. He works as a restaurant dishwasher. Similar lesions are seen on the trunk and left forearm. What is the most likely diagnosis?
➔ Psoriasis (extensor plaques + nail changes → think Psoriasis)
- Note: Skin findings → Salmon-colored plaques with silvery scale over extensor surfaces → Elbows, knees, scalp, forearms.
- Nail involvement → Thickened, pitted nails (common in psoriasis).
- Treated with topical steroids, calcipotriene (vitamin D analog), retinoids, UV phototherapy for mild cases. Biologics (e.g., adalimumab, secukinumab, ustekinumab) and systemic drugs for severe disease
Haley Otman (https://commons.wikimedia.org/wiki/File:Psoriasis_on_elbow.jpg), „Psoriasis on elbow“, https://creativecommons.org/licenses/by/3.0/legalcode
15. A woman presents with a 3-week history of a rash on her left wrist. It appeared after she began wearing a new watch and has persisted despite discontinuing the watch 3 days ago. Exam shows erythematous papules in the wrist area and dry, flaky skin elsewhere. What is the most likely cause of this patient’s symptoms?
➔ Contact dermatitis (localized erythematous rash + recent watch exposure → type IV hypersensitivity)
- Note: Type IV T cell–mediated hypersensitivity reaction.
- Triggers: Nickel (e.g., watch bands), topical antibiotics, poison ivy.
- Rash features:
○ Erythematous, pruritic, papular
○ Often linear or angular, especially with poison ivy. - Treated with avoidance of the allergen and topical corticosteroids.
61cygni (https://commons.wikimedia.org/wiki/File:Shiitake_mushroom_dermatitis.jpg), https://creativecommons.org/licenses/by- sa/4.0/legalcode
16. A 35-year-old patient presents with intensely pruritic, violaceous, polygonal, flat-topped papules distributed symmetrically over the flexor surfaces of the wrists and forearms. On oral examination, the buccal mucosa reveals white, reticulated, lacy streaks. What is the most likely diagnosis?
➔ Lichen planus
- Note: “6 Ps”: Pruritic, Purple, Polygonal, Planar, Papules, and Plaques
- Wickham striae = lacy white pattern on mucosa
- Histology: sawtooth lymphocytic infiltration
- Associated with Hepatitis C.
17. A 17-year-old boy comes to the clinic with worsening facial acne over the past year. Examination shows numerous comedones, inflamed pustules, and papules on the face and upper back. What explains the pathophysiology of this condition?
➔ Acne vulgaris
- Note: Cutibacterium acnes colonizes sebaceous follicles → inflammation, keratin plug formation, comedone development.
- Androgens (puberty, PCOS) → ↑ sebum production → worsens acne
- Risk factors include oily skin, friction (e.g., helmets), occlusive products, and stress.
Treatment
○ Mild acne: topical retinoids, benzoyl peroxide.
○ Moderate acne: add topical antibiotics (clindamycin) or oral antibiotics (tetracyclines, avoid sun exposure → photosensitivity risk).
○ Severe/refractory acne: oral isotretinoin (vitamin A derivative) → requires pregnancy testing due to teratogenicity.
18. A 32-year-old man comes to the office with a 6-month history of chronic diarrhea, unintentional weight loss, and a severely itchy rash. He has a known diagnosis of celiac disease but admits to occasional gluten intake. On exam, he has multiple grouped vesicles and excoriations over the extensor surfaces of his elbows and knees. What is the most likely diagnosis?
➔ Dermatitis herpetiformis
- Note: Autoimmune skin disorder linked to celiac disease presents with pruritic, grouped vesicles on extensor surfaces (elbows, knees, buttocks).
- Pathology shows IgA deposits at dermal papillae, Improves with a gluten-free diet.
19. A 10-year-old child is brought to the emergency department with a 2-day history of high fever, malaise, and painful skin rash. He was recently started on lamotrigine for new-onset epilepsy. Examination reveals widespread erythematous macules with blistering and skin detachment involving the face and trunk. Oral mucosa shows erosions and crusting. What is the most likely diagnosis?
➔ Stevens-Johnson syndrome (SJS)
- Note: Severe CD8+ cytotoxic T-cell–mediated mucocutaneous reaction with widespread epidermal necrosis and sloughing, often due to anticonvulsants (lamotrigine, phenytoin), sulfa drugs
○ SJS <10% Body Surface Area is involved, while TEN >30%.
20. A 51-year-old woman develops painful oral ulcers and flaccid skin blisters that rupture easily with pressure. Biopsy shows intraepidermal acantholysis and antibodies against desmoglein. What is the most likely diagnosis?
➔ Pemphigus vulgaris
- Note: Type II hypersensitivity, IgG antibodies against desmoglein (desmosomes) that normally connect keratinocytes in the epidermis, causing damage → loss of keratinocyte adhesion, and bullae formation.
- Both oral mucosa and skin involvement, flaccid bullae, positive Nikolsky sign with reticular pattern on the immunofluorescence.
- DD. Bullous pemphigoid: IgG antibodies against hemidesmosomes→ Tense blisters that do not rupture, no mucosal involvement. Immunofluorescence shows a linear pattern.
21. A woman presents with hypopigmented patches and macules on her hands and face. The wood lamp accentuates the patches. She has celiac disease controlled with diet and medications. What is the most likely diagnosis?
➔ Vitiligo
- Note: Autoimmune destruction of melanocytes leading to Complete Irregular depigmentation.
- Associated with autoimmune diseases (e.g., thyroid, T1DM, SLE, and celiac disease).
- VS Albinism: hypopigmented skin, hair, and eyelashes.
- VS Tinea versicolor: positive with KOH preparation; hypopigmented patches (no complete depigmentation).
22. A 70-year-old man presents with multiple greasy, yellow, scaly patches on his scalp and nasolabial folds. Diagnosis?
➔ Seborrheic dermatitis
- Note: Inflammatory skin condition affecting sebaceous gland–rich areas (scalp, face, eyebrows) → greasy, yellow scales and erythematous plaques.
- Associated with Parkinson’s disease and HIV in adults, and can present in infants on the scalp as cradle cap.
- Treated with antifungals and mild corticosteroids.
23. A patient presents with a painful ulcer over the distal toe with punched-out edges and decreased pulses. What type of ulcer is this?
➔ Arterial ulcer
- Note: Arterial ulcers = distal toes, punched-out, painful, associated with arterial insufficiency as weak/absent pulses.
- Risk factors: atherosclerosis, diabetes
- Contrast with venous ulcers: mid-calf, around medial malleolus, irregular border, edema, and skin discoloration.
- DM ulcers: painless, over pressure points, with claw toes and absent reflexes.
| Feature | Venous Ulcer | Arterial Ulcer | Neuropathic Ulcer | Pressure Injury |
|---|---|---|---|---|
| Cause | Chronic venous stasis (most common ulcer type) | Peripheral arterial disease (e.g., atherosclerotic narrowing) | Peripheral neuropathy (e.g., diabetic foot ulcers) | Extended pressure without relief (e.g., in immobile patients) |
| Location | Lower leg (gaiter zone, ankle to mid-calf), often near the malleoli | Tips of toes, front of shin, pressure-exposed points | Over bony prominences (e.g., metatarsal heads, heel) | Weight-bearing areas (e.g., sacrum, ischium, heel) |
| Appearance | Irregular edges, shallow, often with exudate | Symmetrical with sharply demarcated, punched-out borders | Thickened (hyperkeratotic) edges, undermined wound borders | Ranges from redness (non-blanchable) to deep skin and tissue loss depending on stage |
| Pain | Mild to moderate discomfort | Severe pain | Typically painless | Painful |
| Associated Features | Varicose veins, skin telangiectasia, leg swelling, stasis dermatitis (red, eczematous patches) | Cool, pale, thin (atrophic) skin; hair loss; absent distal pulses | Claw toe deformities, Charcot joints, loss of reflexes | Commonly complicated by infections or bone involvement (osteomyelitis) |
24. A patient presents with a lump on his upper back. Exam shows a 4-cm mobile, soft, subcutaneous mass with no skin changes. Ultrasound reveals a well-defined, homogenous echotexture. What is the most appropriate next step in management?
➔ Clinical observation
- Note: Lipoma clues → soft, mobile, non-tender, subcutaneous, slow-growing, well-circumscribed mass
- Imaging → Homogeneous echotexture on ultrasound is typical.
- Biopsy is not required, and no treatment is needed.
- Epidermal Inclusion cyst: firm, mobile, with central punctum.
25. A 26-year-old woman presents with painful, red nodules over her shins. She has a chronic cough and dyspnea on exertion. What is the most likely diagnosis?
➔ Erythema nodosum
- Note: It is associated with sarcoidosis, IBD, and streptococcal infection.
James Heilman, MD (https://commons.wikimedia.org/wiki/File:ENlegs.JPG), „ENlegs“, https://creativecommons.org/licenses/by-sa/3.0/legalcode
26. A 28-year-old previously healthy woman presents 2 days after boiling water splashed on her left foot. She developed a painful, red area with blistering on her toes within minutes. Vitals are normal, and she has no systemic symptoms. Physical exam shows erythema and intact bullae on the dorsal foot. What is the most likely diagnosis?
➔ Partial-thickness (second-degree) burn
- Note: Depth of burn classification:
○ Superficial (1st-degree): Painful, dry, red, no blisters
○ Partial-thickness (2nd-degree): Painful, red, blistered, and painful.
○ Full-thickness (3rd-degree): White/charred, painless, leathery - Treatment:
○ Pain control (NSAIDs, acetaminophen)
○ Clean + debride ruptured blisters. Dress with nonadherent gauze + dry covering.
○ Topical antibiotics (e.g., bacitracin, silver sulfadiazine) to prevent infection.
○ Major burn patients require fluid replacement according to the Parkland formula: Total fluid (mL) = 4 × body weight (kg) × %body area burned. 50% of the calculated fluid in 1st 8 hours and the rest 50% in the next 16 hours.
○ Also enteral nutrition should be started in the first 24 hours.
🏁 Final Thoughts
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Disclaimer
Version 1.0 August 2025. No part of this document may be reproduced or distributed without written permission. For personal study only. Medical information herein is for educational purposes and does not constitute clinical advice. Always confirm with current guidelines and your institution’s policies.
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