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Metformin-associated B12 deficiency — USMLE Step 3 MCQ

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EasyInternal Medicine (chronic management)Metformin-associated B12 deficiencyUSMLE Step 3

A 62-year-old woman with type 2 diabetes treated with metformin for 9 years has numb feet and macrocytosis. A1c is 7.1%. B12 level is low and methylmalonic acid is elevated. Which of the following is the most likely explanation for this patient's findings?

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Correct answer: C — Metformin-associated vitamin B12 deficiency

Long-term metformin use is associated with vitamin B12 deficiency and can contribute to neuropathy and macrocytosis.

Reference: ADA Standards of Care in Diabetes 2026