Skin cancer is common in practice and common on the USMLE. These three dermatology practice questions cover the three main types: melanoma, basal cell carcinoma and squamous cell carcinoma, including their histology, risk factors and prognosis. Pick your answer first, then open the explanation.
Question 1: Most important prognostic factor in melanoma
A 52-year-old woman has an asymmetric, 9-mm pigmented lesion on her back with irregular borders and several colors. Excisional biopsy confirms melanoma. Which of the following is the most important prognostic factor for this localized tumor?
A. Diameter of the lesion
B. Number of colors in the lesion
C. Depth of invasion (Breslow thickness)
D. Patient age
E. Location on the trunk
Correct answer: C. Depth of invasion (Breslow thickness)
For localized melanoma, Breslow thickness (measured in millimeters from the granular layer to the deepest tumor cell) is the most important prognostic factor. Deeper tumors are more likely to have entered the vertical growth phase and to metastasize. Ulceration and lymph node status are also important.
Why the other options are wrong:
Diameter and color are part of the ABCDE warning signs, but they do not predict prognosis after diagnosis.
Age and location have a much smaller effect than depth.
High-yield: Melanoma marker: S-100 (also HMB-45, Melan-A). About half of melanomas carry a BRAF V600E mutation, treated with BRAF/MEK inhibitors; immunotherapy uses PD-1 and CTLA-4 inhibitors. Types: superficial spreading (most common), nodular, lentigo maligna, acral lentiginous (most common in darker skin).
Question 2: Pearly papule with rolled borders on the nose
A 70-year-old man has a slowly growing, pearly, flesh-colored papule on the side of his nose with rolled borders, central ulceration and fine blood vessels on the surface. A biopsy is most likely to show which of the following?
A. Keratin pearls and atypical squamous cells
B. Nests of basaloid cells with peripheral palisading
C. Atypical melanocytes at the dermal-epidermal junction
D. Horn cysts within basaloid keratinocytes
E. Noncaseating granulomas
Correct answer: B. Nests of basaloid cells with peripheral palisading
This is basal cell carcinoma (BCC), the most common skin cancer. It appears as a pearly papule with rolled borders and telangiectasias, often with a central ulcer (“rodent ulcer”), usually on the face – classically the upper lip and nose. Histology shows nests of basaloid cells with peripheral palisading and retraction artifact. It invades locally but rarely metastasizes.
High-yield: BCC is linked to mutations in the Hedgehog pathway (PTCH1); advanced cases are treated with vismodegib. Many BCCs suggest Gorlin syndrome (basal cell nevus syndrome). Mohs surgery is used on the face to spare tissue.
Question 3: Skin cancer in a kidney transplant recipient
A 55-year-old man who received a kidney transplant 10 years ago and takes tacrolimus and azathioprine develops a rapidly growing, scaly, ulcerated nodule on the back of his hand. Which of the following skin cancers is most strongly increased in patients like him?
A. Basal cell carcinoma
B. Melanoma
C. Squamous cell carcinoma
D. Merkel cell carcinoma
E. Kaposi sarcoma
Correct answer: C. Squamous cell carcinoma
Organ transplant recipients on long-term immunosuppression have up to a 65–100 times higher risk of squamous cell carcinoma (SCC), and their SCCs are more aggressive. SCC appears as a scaly, keratotic or ulcerated nodule on sun-exposed skin; histology shows atypical keratinocytes invading the dermis with keratin pearls.
Why the other options are wrong:
Basal cell carcinoma also increases, but much less than SCC; in transplant patients the usual BCC-to-SCC ratio is reversed.
Melanoma increases only modestly.
Merkel cell carcinoma and Kaposi sarcoma are increased but are much rarer.
High-yield: Other SCC risk factors: UV exposure, actinic keratosis, chronic wounds and burn scars (Marjolin ulcer), arsenic, HPV and xeroderma pigmentosum. SCC classically affects the lower lip; BCC the upper lip.
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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.
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