Question

Clinical scenario about homocystinuria — which vitamin should be supplemented?

• A. Thiamine
• B. B6
• C. Biotin
• D. Pyruvate

Hint:

The deficient enzyme is cystathionine beta-synthase — think of which vitamin's active form (pyridoxal phosphate) is its coenzyme.

Answer 1

The Correct Option is B

Step 1: Identify the enzyme defect. Classic homocystinuria is most commonly caused by deficiency of cystathionine beta-synthase (CBS), the enzyme that condenses homocysteine with serine to form cystathionine in the trans-sulphuration pathway. The block causes accumulation of homocysteine and methionine.

Step 2: Recall the cofactor. Cystathionine beta-synthase requires pyridoxal phosphate (the active form of vitamin B6) as its coenzyme. Many patients have a pyridoxine-responsive form in which giving high-dose vitamin B6 (pyridoxine) restores residual enzyme activity and lowers homocysteine.

Step 3: Confirm the correct option. Therefore vitamin B6 (pyridoxine) is the vitamin supplemented in homocystinuria; option B is correct. (Folate and B12, which support remethylation of homocysteine to methionine, are often added, but the answer choice tied to CBS is B6.)

Step 4: Exclude the distractors. Thiamine (B1) is the cofactor for pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase and transketolase - relevant to maple syrup urine disease and Wernicke encephalopathy, not homocystinuria; A is wrong. Biotin is the cofactor for carboxylases (e.g. pyruvate carboxylase, propionyl-CoA carboxylase) and is used in biotinidase deficiency, not homocystinuria; C is wrong. Pyruvate is a metabolite, not a vitamin, and is not a treatment for homocystinuria; D is wrong.

Final answer: Option B - B6.