A 40-year-old man with a partial gastrectomy 5 years ago presents with numbness in both feet, imbalance, and memory difficulties. Labs show macrocytic anemia. Serum folate is normal. Serum methylmalonic acid is markedly elevated. Serum homocysteine is also elevated. Which deficiency is most likely?
- A Isolated vitamin B12 deficiency ✓
- B Isolated folate deficiency
- C Combined folate and vitamin B12 deficiency
- D Isolated iron deficiency
Correct answer: A. Isolated vitamin B12 deficiency
Explanation
Elevated methylmalonic acid (MMA) is specific to vitamin A12 deficiency, as methylmalonyl-CoA mutase requires adenosylcobalamin. Folate deficiency causes only elevated homocysteine, not MMA. The neurological symptoms (posterior column involvement, subacute combined degeneration) also point to A12 deficiency. Normal folate rules out combined deficiency. The partial gastrectomy history impairs intrinsic factor production, causing malabsorption of A12.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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