You are answering a USMLE-style medical question. Question: A 42-year-old chronic alcoholic man was admitted to the hospital for inappropriate behavior and disturbed memory. He presents with severe retrograde memory loss, confusion, and confabulation. Neurologic examination showed horizontal nystagmus. He also has bilateral pretibial pitting edema and perioral erythema. CT studies of the brain were normal. The duty physician suspects the patient may be vitamin deficient. Which of the following reactions does the deficient vitamin mediate? Options: A) Alpha-Ketoglutarate + NAD+ + CoA <=> Succinyl-CoA + CO2 + NADH B) Succinate + FAD (enzyme bound) <=> Fumarate + FADH2 C) Isocitrate + NAD+ <=> Alpha-Ketoglutarate + CO2 + NADH D) Succinyl-CoA + Pi + GDP <=> Succinate + GTP + CoA Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.