Dermatology questions appear throughout USMLE Step 1, usually as a short description of a lesion that you must connect to its pathology. These three USMLE dermatology practice questions, written by a dermatologist, cover classic lesions you must recognize: seborrheic keratosis, actinic keratosis and psoriasis. Pick your answer first, then open the explanation.
Question 1: Sudden appearance of many “stuck-on” skin lesions
A 68-year-old man notices dozens of new, itchy, brown, waxy, “stuck-on” papules on his back that appeared over 2 months. He has also lost 7 kg (15 lb) and feels full after small meals. Which of the following is the most appropriate next step?
A. Reassure him that these are benign age-related lesions B. Remove the lesions with cryotherapy C. Evaluate for an internal malignancy, especially of the GI tract D. Start topical corticosteroids
Correct answer: C. Evaluate for an internal malignancy, especially of the GI tract
Single seborrheic keratoses are common and benign. But the sudden eruption of many seborrheic keratoses is the sign of Leser-Trélat, a paraneoplastic sign most often linked to gastric adenocarcinoma and other GI cancers. Weight loss and early satiety make a gastric tumor likely here, so he needs upper endoscopy.
Why the other options are wrong:
Reassurance would be correct for a few longstanding lesions, but not for a sudden eruption with weight loss.
Cryotherapy treats individual lesions for cosmetic reasons; it misses the underlying cancer.
Topical steroids have no role in seborrheic keratosis.
High-yield: Histology of seborrheic keratosis: proliferation of basaloid keratinocytes with keratin-filled horn cysts. Other GI-cancer skin signs: acanthosis nigricans (gastric adenocarcinoma) and Trousseau syndrome (pancreatic cancer).
Question 2: Rough, scaly patches on sun-exposed skin
A 62-year-old fair-skinned farmer has several rough, gritty, scaly patches on his forehead and the backs of his hands. They are easier to feel than to see. Which of the following is the most important complication of these lesions?
A. Basal cell carcinoma B. Melanoma C. Squamous cell carcinoma D. Keratoacanthoma E. Seborrheic keratosis
Correct answer: C. Squamous cell carcinoma
These are actinic (solar) keratoses: premalignant lesions caused by chronic UV damage. Histology shows atypical keratinocytes in the lower epidermis with parakeratosis. A small percentage progress to invasive squamous cell carcinoma; the risk rises with the number of lesions.
Why the other options are wrong:
Basal cell carcinoma arises de novo and does not develop from actinic keratosis.
Melanoma arises from melanocytes, not keratinocytes.
Keratoacanthoma is a rapidly growing, dome-shaped nodule with a central keratin plug; it is not the expected outcome of actinic keratosis.
Seborrheic keratosis is a benign, unrelated lesion.
High-yield: Treatment: cryotherapy for few lesions; topical 5-fluorouracil or imiquimod for many (field therapy). Actinic cheilitis is the same process on the lower lip.
Question 3: Silvery plaques that bleed when scraped
A 30-year-old man has well-demarcated, salmon-pink plaques with silvery scale on his elbows and knees, and pitting of his fingernails. Scraping the scale produces pinpoint bleeding. A biopsy is most likely to show which of the following?
A. Acantholysis of keratinocytes above the basal layer B. Parakeratosis with neutrophil collections in the stratum corneum C. Subepidermal blister with eosinophils D. Spongiosis (intercellular edema of the epidermis) E. Noncaseating granulomas in the dermis
Correct answer: B. Parakeratosis with neutrophil collections in the stratum corneum
This is psoriasis. Histology shows acanthosis with elongated rete ridges, parakeratosis (nuclei retained in the stratum corneum), Munro microabscesses (neutrophils in the stratum corneum), a thinned or absent granular layer, and thinning of the epidermis over dilated dermal papillae. That thinning explains the pinpoint bleeding when scale is removed (Auspitz sign).
Why the other options are wrong:
Suprabasal acantholysis is pemphigus vulgaris.
Subepidermal blister with eosinophils is bullous pemphigoid.
Spongiosis is the hallmark of eczematous dermatitis.
Noncaseating granulomas suggest sarcoidosis.
High-yield: Psoriasis is driven by the Th17/IL-17 and IL-23 pathway. New plaques at sites of injury = Koebner phenomenon. Associations: psoriatic arthritis (pencil-in-cup deformity, HLA-B27). Biologics: TNF-α, IL-17 and IL-23 inhibitors.
Key takeaways
Sudden eruption of many seborrheic keratoses → sign of Leser-Trélat → look for GI cancer.
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Written and reviewed by Dr. Ray Makar, MD, dermatologist with 20 years of clinical practice. Last reviewed: October 2026. For exam preparation only, not medical advice.
