If a patient presents with fatigue and pallor several weeks after starting a strict vegan diet, which mechanism explains why vitamin B12 deficiency can cause macrocytic anemia?

💡 Explanation

Vitamin B12 is essential for DNA synthesis in rapidly dividing cells, including erythroblasts. In B12 deficiency, DNA synthesis is impaired, and erythroblasts cannot divide properly, resulting in large, immature red blood cells (macrocytes) being released into the circulation; therefore, impaired DNA synthesis explains macrocytic anemia rather than increased apoptosis or accelerated erythropoiesis.

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