Macrocytic anemia is characterized by macrocytosis due to which deficiencies?

• A. It is characterized by microcytosis due to iron deficiency
• B. Macrocytosis due to folic acid or vitamin B12 deficiency
• C. Macrocytosis due to niacin deficiency
• D. Macrocytosis is not affected by vitamin deficiencies

Answer: (B)

Macrocytic anemia arises when red blood cells are larger than normal because DNA synthesis in erythroid precursors is impaired. The classic nutritional causes are folate (folic acid) and vitamin B12 deficiencies. Without enough folate or B12, thymidine synthesis is disrupted, so nuclear maturation lags behind cytoplasmic growth. This creates megaloblastic erythropoiesis with oversized red cells (macrocytosis) and often hypersegmented neutrophils on the smear. Niacin deficiency isn’t a typical driver of macrocytosis, and iron deficiency tends to cause microcytosis (smaller cells). So macrocytosis points to folate or B12 deficiency as the best explanation.