Nucleotide metabolism: hyperuricaemia in glycogen storage disease type I: 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 · Nucleotide metabolism: hyperuricaemia in glycogen storage disease type I

Sample item

A 2-year-old with fasting hypoglycaemia, hepatomegaly and lactic acidosis has a serum urate of 11 mg/dL. Which mechanism contributes to the hyperuricaemia?

AAbsent xanthine oxidase leads to xanthine build-up in bloodBLactate competes with urate for renal secretion, and purine synthesis risesCIncreased URAT1 activity raises urate reabsorption in the gutDDefective HGPRT leaves hypoxanthine unsalvaged and raises PRPP

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

The answer and the reasoning

B. Lactate competes with urate for renal secretion, and purine synthesis rises

In glucose-6-phosphatase deficiency, excess glucose-6-phosphate is shunted to the pentose phosphate pathway, increasing ribose 5-phosphate and PRPP and so purine synthesis. Lactate also competes with urate for tubular secretion, so excretion falls.

Option by option

A. Absent xanthine oxidase leads to xanthine build-up in blood
Xanthine oxidase deficiency lowers urate and raises xanthine, the opposite of this picture.
B. Lactate competes with urate for renal secretion, and purine synthesis rises (correct)
Correct. Both increased production and reduced renal clearance of urate occur.
C. Increased URAT1 activity raises urate reabsorption in the gut
URAT1 acts in the renal tubule, not the gut, and the drug-sensitive route is not the issue here.
D. Defective HGPRT leaves hypoxanthine unsalvaged and raises PRPP
HGPRT deficiency is Lesch-Nyhan syndrome with neurological signs, not hypoglycaemia and hepatomegaly.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Lactate competes with urate for renal secretion, and purine synthesis rises. In glucose-6-phosphatase deficiency, excess glucose-6-phosphate is shunted to the pentose phosphate pathway, increasing ribose 5-phosphate and PRPP and so purine synthesis.

Why is "Absent xanthine oxidase leads to xanthine build-up in blood" not the answer?

Xanthine oxidase deficiency lowers urate and raises xanthine, the opposite of this picture.

Why is "Increased URAT1 activity raises urate reabsorption in the gut" not the answer?

URAT1 acts in the renal tubule, not the gut, and the drug-sensitive route is not the issue here.

Why is "Defective HGPRT leaves hypoxanthine unsalvaged and raises PRPP" not the answer?

HGPRT deficiency is Lesch-Nyhan syndrome with neurological signs, not hypoglycaemia and hepatomegaly.

What concept does this question test?

It tests Nucleotide metabolism: hyperuricaemia in glycogen storage disease type I in Biochemistry. Lactate also competes with urate for tubular secretion, so excretion falls.

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 Nucleotide metabolism: hyperuricaemia in glycogen storage disease type I?

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.