A 60-year-old female presents with progressive fatigue and exertional dyspnea. Peripheral blood smear reveals macro-ovalocytes and hypersegmented neutrophils (neutrophils with 6 or more nuclear lobes). Serum methylmalonic acid and homocysteine levels are both markedly elevated. What is the diagnosis?

Answer and explanation

Correct answer: C. Vitamin B12 (cobalamin) deficiency

  1. Step 1: Differentiate megaloblastic anemias: Both B12 and folate deficiency cause macrocytic anemia with hypersegmented neutrophils and elevated homocysteine.
  2. Step 2: However, only vitamin B12 deficiency leads to impaired conversion of methylmalonyl-CoA to succinyl-CoA, resulting in elevated methylmalonic acid (MMA). In folate deficiency, MMA is normal.
Answer
Vitamin B12 (cobalamin) deficiency.
Why not the others: 'Folate deficiency' presents with elevated homocysteine but strictly normal methylmalonic acid levels; 'Iron deficiency' presents with microcytic hypochromic red cells; 'Aplastic anemia' causes pancytopenia with hypocellular bone marrow.

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