Which statement correctly describes megaloblastic anemia in B9 and B12 deficiencies?

• A. Both can cause megaloblastic anemia with macrocytosis
• B. Only B12 can cause megaloblastic anemia
• C. Only B9 can cause macrocytosis
• D. Neither causes macrocytosis

Answer: (A)

Megaloblastic anemia with macrocytosis occurs when DNA synthesis in rapidly dividing cells is impaired because of insufficient folate (B9) or B12. Folate provides the thymidylate synthesis needed for DNA replication, while B12 acts as a cofactor in regenerating usable folate for DNA synthesis. When either nutrient is deficient, developing erythroid cells cannot mature nuclei quickly enough, so the cells become large (macrocytic) and form megaloblasts in the bone marrow. This is true for both B9 and B12 deficiencies, which is why the statement that both can cause megaloblastic anemia with macrocytosis is correct. Clinically, B12 deficiency may also involve neurologic symptoms from demyelination and elevated methylmalonic acid, whereas folate deficiency typically lacks those neurologic features, though the hematologic pattern of macrocytosis can still be present in either deficiency.