The skin often gives the first clue to a disease inside the body, and the USMLE loves these connections. These three dermatology practice questions cover acanthosis nigricans, erythema nodosum and dermatomyositis. Pick your answer first, then open the explanation.
Question 1: Velvety dark skin on the neck of an obese teenager
A 15-year-old obese girl has thick, velvety, darkly pigmented skin on the back of her neck and in her armpits. She also has irregular periods and acne. Which of the following is the most likely underlying cause of her skin finding?
A. Addison disease B. Insulin resistance C. Hemochromatosis D. Poor hygiene E. Gastric adenocarcinoma
Correct answer: B. Insulin resistance
This is acanthosis nigricans. In young, overweight patients it is almost always caused by insulin resistance: high insulin levels stimulate keratinocytes and fibroblasts through IGF-1 receptors. Her irregular periods and acne suggest polycystic ovary syndrome, which is also driven by insulin resistance.
Why the other options are wrong:
Addison disease causes diffuse hyperpigmentation, especially of creases, scars and mucosa, but not velvety thickening.
Hemochromatosis causes a bronze skin color.
Poor hygiene is a common misconception; the pigment does not wash off.
Gastric adenocarcinoma causes malignant acanthosis nigricans, but in older, often thin patients with sudden, extensive lesions.
High-yield: Screen patients with acanthosis nigricans for type 2 diabetes (HbA1c or fasting glucose). Sudden, widespread acanthosis nigricans in an older adult, especially with lesions on the palms (“tripe palms”) or mouth, suggests internal cancer.
Question 2: Tender red nodules on the shins
A 32-year-old woman has tender, warm, red nodules on both shins for 2 weeks, along with fever and joint pain. A chest X-ray shows bilateral hilar lymphadenopathy. Which of the following is the most likely diagnosis?
A. Erythema multiforme B. Erythema nodosum due to sarcoidosis C. Cellulitis D. Pyoderma gangrenosum E. Superficial thrombophlebitis
Correct answer: B. Erythema nodosum due to sarcoidosis
Erythema nodosum is a septal panniculitis (inflammation of the fat septa) that causes tender red nodules, usually on the shins, which heal without scarring or ulceration. Erythema nodosum + bilateral hilar lymphadenopathy + fever and arthritis = Löfgren syndrome, an acute form of sarcoidosis with a good prognosis.
Why the other options are wrong:
Erythema multiforme causes target lesions, usually after herpes simplex infection.
Cellulitis is usually one-sided, spreading and not linked to hilar adenopathy.
Pyoderma gangrenosum forms painful ulcers with violaceous borders, linked to inflammatory bowel disease.
Question 3: Purple eyelids and red papules over the knuckles
A 58-year-old woman has a violet rash on her upper eyelids, red scaly papules over her knuckles, and difficulty climbing stairs and combing her hair. Creatine kinase is elevated. In addition to starting treatment, which of the following is the most important next step?
A. Thyroid function tests B. Screening for an underlying malignancy C. Skin patch testing D. Lumbar puncture E. Nerve conduction studies only
Correct answer: B. Screening for an underlying malignancy
This is dermatomyositis: heliotrope rash (violet eyelids), Gottron papules (over the knuckles) and symmetric proximal muscle weakness with high CK. In adults, dermatomyositis is a paraneoplastic syndrome in a significant number of cases, most often with ovarian, lung, gastric and colorectal cancers, so age-appropriate cancer screening is essential.
Why the other options are wrong:
Thyroid tests can be checked for weakness, but they do not explain the rash and are not the key step.
Patch testing is for allergic contact dermatitis.
Lumbar puncture has no role.
Nerve conduction studies do not address the cancer risk.
High-yield: Antibodies: anti-Mi-2 (classic, good prognosis), anti-Jo-1 (antisynthetase syndrome with interstitial lung disease), anti-TIF1-γ (strongly linked to cancer). Biopsy: perifascicular atrophy (CD4+ cells around fascicles). Treat with corticosteroids.
Key takeaways
Acanthosis nigricans → insulin resistance (young/obese) or GI cancer (older, sudden).
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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.
