Amino acid metabolism: cysteine in cystathionine synthase deficiency: 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: cysteine in cystathionine synthase deficiency

Sample item

A boy with classic homocystinuria is placed on a methionine-restricted diet. Which additional amino acid must be supplied because the metabolic block prevents its synthesis from methionine?

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

The answer and the reasoning

A. Cysteine

Cysteine is made from methionine-derived homocysteine through cystathionine beta-synthase and cystathionine gamma-lyase. When the first enzyme is blocked, cysteine cannot be made, so it becomes conditionally essential. The same logic makes tyrosine conditionally essential in phenylketonuria.

Option by option

  • A. Cysteine (correct)

    Correct. Cysteine synthesis depends on the blocked transsulfuration pathway, so it becomes conditionally essential.

  • B. Arginine

    Arginine is made through the urea cycle, which is not involved.

  • C. Tyrosine

    Tyrosine is made from phenylalanine, a pathway unaffected here.

  • D. Glycine

    Glycine is made from serine and is not dependent on homocysteine.

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, Cysteine. Cysteine is made from methionine-derived homocysteine through cystathionine beta-synthase and cystathionine gamma-lyase.

Why is "Arginine" not the answer?

Arginine is made through the urea cycle, which is not involved.

Why is "Tyrosine" not the answer?

Tyrosine is made from phenylalanine, a pathway unaffected here.

Why is "Glycine" not the answer?

Glycine is made from serine and is not dependent on homocysteine.

What concept does this question test?

It tests Amino acid metabolism: cysteine in cystathionine synthase deficiency in Biochemistry. When the first enzyme is blocked, cysteine cannot be made, so it becomes conditionally essential. The same logic makes tyrosine conditionally essential in phenylketonuria.

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: cysteine in cystathionine synthase deficiency?

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.