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MBBS QUESTION #10330
Question 1
A strict vegan without B12 supplementation develops macrocytic anemia together with subacute combined degeneration of the spinal cord (affecting both dorsal columns and corticospinal tracts). This particular combination of hematological and neurological findings is characteristic of deficiency of:
  • Folate (vitamin B9) alone, which also produces identical neurological findings
  • Vitamin B12 (cobalamin), required both for DNA synthesis (via the methionine synthase reaction) and for myelin maintenance (via the methylmalonyl-CoA mutase reaction)✔️
  • Vitamin B6 (pyridoxine)
  • Vitamin C (ascorbic acid)
Correct Answer Explanation
Both folate and B12 deficiency cause megaloblastic anemia via impaired DNA synthesis, but only B12 deficiency additionally impairs the methylmalonyl-CoA to succinyl-CoA conversion, disrupting myelin lipid metabolism and producing the distinctive neurological picture of subacute combined degeneration — an important reason not to treat suspected B12 deficiency with folate alone, which can mask the anemia while neurological damage progresses.