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B12 deficiency — USMLE Step 3 MCQ

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ModerateAmbulatory Medicine / Primary CareB12 deficiencyUSMLE Step 3

A 69-year-old man has progressive numbness in both feet and difficulty with balance. Hemoglobin is 10.2 g/dL, mean corpuscular volume is 112 fL, and serum methylmalonic acid is elevated. He eats meat. Anti-intrinsic factor antibody is positive. Which of the following is the most likely explanation for this patient's findings?

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Correct answer: A — Autoimmune loss of gastric intrinsic factor

Pernicious anemia causes vitamin B12 deficiency due to autoimmune intrinsic factor deficiency, leading to macrocytic anemia and neurologic findings.

Reference: American Society of Hematology Review: Vitamin B12 Deficiency