Question:easy
Clinical scenario about homocystinuria — which vitamin should be supplemented?
Clinical scenario about homocystinuria — which vitamin should be supplemented?
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The deficient enzyme is cystathionine beta-synthase — think of which vitamin's active form (pyridoxal phosphate) is its coenzyme.
Updated On: Jun 22, 2026
- Thiamine
- B6
- Biotin
- Pyruvate
Show Solution
The Correct Option is B
Solution and Explanation
Trace the metabolic pathway. Homocystinuria results from a block in the trans-sulphuration of methionine. The key enzyme, cystathionine beta-synthase (CBS), converts homocysteine + serine into cystathionine. When CBS is deficient, homocysteine builds up, producing the classic features: ectopia lentis (downward lens dislocation), marfanoid habitus, mental retardation and thromboembolism.
Match the enzyme to its vitamin cofactor. CBS uses pyridoxal-5'-phosphate, the active coenzyme form of vitamin B6 (pyridoxine), to work. In the common pyridoxine-responsive variant, large doses of B6 boost the residual enzyme and bring homocysteine down - making B6 the supplement that is asked about.
Why not the others.
- Thiamine (B1): cofactor for pyruvate/alpha-ketoglutarate dehydrogenase and transketolase; its disorders are MSUD and Wernicke, not homocystinuria.
- Biotin: cofactor for carboxylase enzymes; used in biotinidase deficiency.
- Pyruvate: an intermediary metabolite, not a vitamin and not a therapy here.
Hence the vitamin to supplement in homocystinuria is B6 (Option B). Folate and B12 may be co-administered to aid remethylation, but the cofactor of the deficient CBS enzyme is B6.
Match the enzyme to its vitamin cofactor. CBS uses pyridoxal-5'-phosphate, the active coenzyme form of vitamin B6 (pyridoxine), to work. In the common pyridoxine-responsive variant, large doses of B6 boost the residual enzyme and bring homocysteine down - making B6 the supplement that is asked about.
Why not the others.
- Thiamine (B1): cofactor for pyruvate/alpha-ketoglutarate dehydrogenase and transketolase; its disorders are MSUD and Wernicke, not homocystinuria.
- Biotin: cofactor for carboxylase enzymes; used in biotinidase deficiency.
- Pyruvate: an intermediary metabolite, not a vitamin and not a therapy here.
Hence the vitamin to supplement in homocystinuria is B6 (Option B). Folate and B12 may be co-administered to aid remethylation, but the cofactor of the deficient CBS enzyme is B6.
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