Macrocytic anemia is most commonly associated with deficiency of which vitamin?

Macrocytic anemia is most commonly associated with deficiency of which vitamin?

• A. Iron
• B. Vitamin B12
• C. Vitamin C
• D. Vitamin D

Answer: (B)

Macrocytic anemia happens when red cell precursors in the bone marrow can’t complete DNA synthesis quickly enough, so the cells grow larger before dividing. This is typically seen when vitamins needed for DNA production are deficient. Vitamin B12 is a crucial cofactor in reactions that regenerate tetrahydrofolate and support thymidine synthesis, both essential for DNA replication. When B12 is lacking, DNA synthesis slows, marrow cells become megaloblastic, and larger red cells are released into the circulation, producing macrocytosis. In adults, B12 deficiency often comes from pernicious anemia, insufficient dietary intake (especially in strict vegans), or absorption problems, and it can be accompanied by neurologic symptoms due to demyelination. Iron deficiency, by contrast, leads to microcytic, not macrocytic, anemia. Vitamin C deficiency does not characteristically produce macrocytosis and is associated with scurvy symptoms. Vitamin D deficiency affects bone and calcium metabolism rather than red blood cell size. Thus, deficiency of vitamin B12 best explains macrocytic anemia among the options.

Macrocytic anemia happens when red cell precursors in the bone marrow can’t complete DNA synthesis quickly enough, so the cells grow larger before dividing. This is typically seen when vitamins needed for DNA production are deficient. Vitamin B12 is a crucial cofactor in reactions that regenerate tetrahydrofolate and support thymidine synthesis, both essential for DNA replication. When B12 is lacking, DNA synthesis slows, marrow cells become megaloblastic, and larger red cells are released into the circulation, producing macrocytosis. In adults, B12 deficiency often comes from pernicious anemia, insufficient dietary intake (especially in strict vegans), or absorption problems, and it can be accompanied by neurologic symptoms due to demyelination.

Iron deficiency, by contrast, leads to microcytic, not macrocytic, anemia. Vitamin C deficiency does not characteristically produce macrocytosis and is associated with scurvy symptoms. Vitamin D deficiency affects bone and calcium metabolism rather than red blood cell size. Thus, deficiency of vitamin B12 best explains macrocytic anemia among the options.