If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 2 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.
A 50-year-old alcoholic man with chronic hepatitis C infection is brought to the emergency department by the police, because he has blisters and crusted lesions on sun-exposed skin of his face and lower arms. Plasma porphyrins are elevated, and follow-up studies demonstrate elevated uroporphyrin I in urine and isocoproporphyrin in feces. A biopsy of one of the lesions reveals subepidermal blisters with minimal inflammation, thickening of vessel walls in the papillary dermis, marked solar elastosis, and “caterpillar bodies” in the roof of the blister.
Which of the following is the most likely diagnosis?
A. Acute intermittent porphyria.
B. Delta-aminolevulinic acid dehydratase deficiency.
C. Erythropoietic protoporphyria.
D. Hereditary coproporphyria.
E. Porphyria cutanea tarda.
The correct answer is E.
Porphyria cutanea tarda is the most common of all of the porphyrias and is consequently a likely target on the USMLE. It causes chronic blistering and crusting lesions on sun-exposed skin. The defective enzyme in heme synthesis is uroporphyrinogen decarboxylase. Precipitating factors include iron (even in normal amounts in some cases), estrogen use, alcohol use, and chronic hepatitis C infection. Skin biopsy can be helpful but is usually not completely specific (the “caterpillar bodies” in the question stem are clumps of basement membrane material). Porphyrin analyses demonstrate the findings in the question stem.
Choice A: Acute intermittent porphyria is one of the more common forms of porphyria and typically presents with severe abdominal pain.
Choice B: Delta-aminolevulinic acid dehydratase deficiency is a rare form of porphyria that can cause abdominal pain and hemolysis.
Choice C: Erythropoietic porphyria is one of the more common forms of porphyria and typically presents in childhood or infancy with acute, rather than chronic, photosensitivity with pain and swelling after sunlight exposure.
Choice D: Hereditary coproporphyria is a rare porphyria than presents with abdominal pain.
For more prep questions on USMLE Steps 1, 2 and 3, view other posts in this series.
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