Purine salvage: hypoxanthine-guanine phosphoribosyltransferase: 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 · Purine salvage: hypoxanthine-guanine phosphoribosyltransferase

Sample item

Fibroblasts from a boy with urate overproduction show a markedly raised intracellular PRPP concentration and an increased rate of de novo purine synthesis. Which enzyme is absent?

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

The answer and the reasoning

B. Hypoxanthine-guanine phosphoribosyltransferase

Without the salvage enzyme PRPP is not consumed in recycling guanine and the hypoxanthine base, so it accumulates and drives amidophosphoribosyltransferase. The result is overproduction of purines and urate. Salvage therefore restrains de novo synthesis in a healthy cell.

Option by option

  • A. Adenine phosphoribosyltransferase

    Adenine phosphoribosyltransferase deficiency causes 2,8-dihydroxyadenine stones and does not raise urate.

  • B. Hypoxanthine-guanine phosphoribosyltransferase (correct)

    Correct. Failure of salvage leaves PRPP unconsumed and de novo purine synthesis runs unchecked.

  • C. Adenosine deaminase

    Adenosine deaminase deficiency accumulates deoxyadenosine in lymphocytes and lowers urate.

  • D. Xanthine oxidase

    Xanthine oxidase deficiency reduces urate formation and does not raise PRPP.

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 B, Hypoxanthine-guanine phosphoribosyltransferase. Without the salvage enzyme PRPP is not consumed in recycling guanine and the hypoxanthine base, so it accumulates and drives amidophosphoribosyltransferase.

Why is "Adenine phosphoribosyltransferase" not the answer?

Adenine phosphoribosyltransferase deficiency causes 2,8-dihydroxyadenine stones and does not raise urate.

Why is "Adenosine deaminase" not the answer?

Adenosine deaminase deficiency accumulates deoxyadenosine in lymphocytes and lowers urate.

Why is "Xanthine oxidase" not the answer?

Xanthine oxidase deficiency reduces urate formation and does not raise PRPP.

What concept does this question test?

It tests Purine salvage: hypoxanthine-guanine phosphoribosyltransferase in Biochemistry. The result is overproduction of purines and urate. Salvage therefore restrains de novo synthesis in a healthy cell.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 Purine salvage: hypoxanthine-guanine phosphoribosyltransferase?

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.