Consider the metabolic fate of homocysteine: synthesize a clear explanation which single vitamin deficiency (from the B-complex) would most likely cause elevated homocysteine with normal methylmalonic acid (MMA) and why.

• A. Folate deficiency always elevates MMA without changing homocysteine, so it cannot explain elevated homocysteine with normal MMA.
• B. Vitamin B12 deficiency, because B12 specifically blocks the transsulfuration step leading to isolated homocysteine elevation without affecting MMA.
• C. Vitamin B6 deficiency, because B6-dependent enzymes are required for the transsulfuration pathway that converts homocysteine to cysteine; deficiency raises homocysteine while MMA remains normal.
• D. Niacin deficiency, because NAD+ shortage directly increases homocysteine synthesis while leaving MMA unaffected.

Answer: (C)