Amino acid metabolism: betaine in homocystinuria: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Amino acid metabolism: betaine in homocystinuria

Sample item

A child with B6-nonresponsive homocystinuria is given betaine in addition to a methionine-restricted diet. How does betaine lower plasma homocysteine?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. It donates a methyl group to remethylate homocysteine to methionine

Betaine-homocysteine methyltransferase in liver uses betaine (trimethylglycine) as a methyl donor to convert homocysteine back to methionine. This pathway is independent of folate and vitamin B12, so it bypasses the usual remethylation route. Methionine may rise, so levels are monitored.

Option by option

  • A. It donates a methyl group to remethylate homocysteine to methionine (correct)

    Correct. Betaine is a methyl donor for the alternative remethylation pathway in liver.

  • B. It activates cystathionine beta-synthase directly

    Betaine does not activate the synthase, which is the defective enzyme in classic disease.

  • C. It supplies the cofactor pyridoxal phosphate

    Pyridoxal phosphate comes from pyridoxine therapy, which this child did not respond to.

  • D. It converts homocysteine to cysteine through transsulfuration

    Transsulfuration is blocked in this disorder and betaine does not restore it.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, It donates a methyl group to remethylate homocysteine to methionine. Betaine-homocysteine methyltransferase in liver uses betaine (trimethylglycine) as a methyl donor to convert homocysteine back to methionine.

Why is "It activates cystathionine beta-synthase directly" not the answer?

Betaine does not activate the synthase, which is the defective enzyme in classic disease.

Why is "It supplies the cofactor pyridoxal phosphate" not the answer?

Pyridoxal phosphate comes from pyridoxine therapy, which this child did not respond to.

Why is "It converts homocysteine to cysteine through transsulfuration" not the answer?

Transsulfuration is blocked in this disorder and betaine does not restore it.

What concept does this question test?

It tests Amino acid metabolism: betaine in homocystinuria in Biochemistry. This pathway is independent of folate and vitamin B12, so it bypasses the usual remethylation route. Methionine may rise, so levels are monitored.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Amino acid metabolism: betaine in homocystinuria?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.