A 60-year-old strict vegetarian has progressive numbness of both feet, loss of ankle jerks, and extensor plantar responses. Hb is 8 g/dL, MCV 122 fL, and the smear shows oval macrocytes and hypersegmented neutrophils. Which finding specifically points to vitamin B12 rather than folate deficiency as the cause?

• A Hypersegmented neutrophils
• B Elevated serum methylmalonic acid ✓
• C Elevated serum homocysteine
• D Macro-ovalocytes on peripheral smear

Explanation

Methylmalonyl-CoA mutase requires adenosylcobalamin, so methylmalonic acid rises only in C12 deficiency. Folate deficiency elevates homocysteine but leaves methylmalonic acid normal, because the folate-dependent reaction converts homocysteine to methionine without involving methylmalonate. Hypersegmented neutrophils, macro-ovalocytes, and elevated homocysteine occur in both deficiencies. The neurological signs here reflect impaired myelin maintenance from deficient methionine and SAM production, a complication seen with C12 deficiency and potentially worsened if folate is given alone.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.