A 58-year-old man has been on metformin 2 g daily for eight years and now presents with macrocytic anemia. Peripheral smear shows hypersegmented neutrophils and his serum methylmalonic acid level is elevated. The most likely explanation is:
- A Metformin-induced interference with intrinsic factor secretion
- B Metformin-associated malabsorption of vitamin B12 from the terminal ileum ✓
- C Direct bone marrow toxicity of metformin
- D Metformin-accelerated degradation of folate stores
Correct answer: B. Metformin-associated malabsorption of vitamin B12 from the terminal ileum
Explanation
Long-term metformin use impairs calcium-dependent uptake of the B12-intrinsic factor complex in the distal ileum, leading to deficiency in up to a third of chronic users. Elevated methylmalonic acid confirms B12 rather than folate deficiency. Supplementation corrects the anemia without stopping metformin. Intrinsic factor production is unaffected, marrow toxicity is not described, and folate metabolism is not the target.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.