A 55-year-old male with chronic alcoholism and paresthesias has a macrocytic anemia with MCV of 115 fL and hypersegmented neutrophils. What biochemical marker is elevated in B12 deficiency but normal in folate deficiency?
Answer and explanation
Correct answer: D. Serum methylmalonic acid
- Step 1: Both vitamin B12 (cobalamin) and folate deficiencies cause megaloblastic macrocytic anemia and elevated serum homocysteine.
- Step 2: Vitamin B12 is an essential cofactor for methylmalonyl-CoA mutase; therefore, elevated serum methylmalonic acid specifically distinguishes B12 deficiency from folate deficiency.
Answer
Serum methylmalonic acid.
Why not the others: Serum homocysteine is elevated in both vitamin B12 and folate deficiencies due to impaired methionine synthase activity. Serum ferritin reflects total body iron storage and is unaffected by cobalamin metabolism. The reticulocyte index is suppressed in both megaloblastic states due to ineffective erythropoiesis.
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