Which of the following is a common cause of megaloblastic anemia?

• A. Iron deficiency anemia
• B. Sideroblastic anemia
• C. Aplastic anemia
• D. Vitamin B12 deficiency

Answer: (D)

Megaloblastic anemia happens when DNA synthesis in rapidly dividing cells, like developing red blood cells, is impaired. That delay in nuclear maturation leads to oversized, immature cells and a macrocytic blood picture, often with hypersegmented neutrophils on smear.

Vitamin B12 is essential for regenerating tetrahydrofolate in the folate cycle. Without adequate B12, folate becomes trapped and unavailable for making thymidine, a building block of DNA. This disruption specifically halts DNA synthesis in erythroid precursors, producing the characteristic megaloblastic changes.

Other conditions listed don’t primarily cause this DNA-synthesis defect. Iron deficiency and sideroblastic anemia are microcytic or normocytic and reflect problems with heme synthesis or iron utilization, not impaired DNA replication. Aplastic anemia involves bone marrow failure with pancytopenia rather than the macrocytosis and abnormal maturation seen in megaloblastic anemia.

Thus, vitamin B12 deficiency best explains the megaloblastic pattern.