Amino acid metabolism: cystinuria treatment: 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: cystinuria treatment

Sample item

A man with recurrent cystine stones continues to form them despite high fluid intake and urinary alkalinisation. A thiol drug that forms a more soluble mixed disulfide with cysteine is added. Which drug is this?

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

The answer and the reasoning

A. Tiopronin

Cystine is poorly soluble, especially at acid pH. Tiopronin and penicillamine contain a thiol group that reacts with cysteine by disulfide exchange, forming a mixed disulfide that is much more soluble. High fluid intake and alkalinisation are first-line measures.

Option by option

  • A. Tiopronin (correct)

    Correct. Tiopronin forms soluble mixed disulfides, reducing free cystine.

  • B. Allopurinol

    Allopurinol lowers uric acid production and does not alter cystine solubility.

  • C. Hydrochlorothiazide

    Thiazides lower urinary calcium and are used for calcium stones, with no effect on cystine.

  • D. Probenecid

    Probenecid increases renal urate excretion and does not help cystine.

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, Tiopronin. Cystine is poorly soluble, especially at acid pH.

Why is "Allopurinol" not the answer?

Allopurinol lowers uric acid production and does not alter cystine solubility.

Why is "Hydrochlorothiazide" not the answer?

Thiazides lower urinary calcium and are used for calcium stones, with no effect on cystine.

Why is "Probenecid" not the answer?

Probenecid increases renal urate excretion and does not help cystine.

What concept does this question test?

It tests Amino acid metabolism: cystinuria treatment in Biochemistry. Tiopronin and penicillamine contain a thiol group that reacts with cysteine by disulfide exchange, forming a mixed disulfide that is much more soluble. High fluid intake and alkalinisation are first-line measures.

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: cystinuria treatment?

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.