Amino acid metabolism: lysinuric protein intolerance: 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: lysinuric protein intolerance

Sample item

A 3-year-old avoids protein-rich food and has growth failure, hepatosplenomegaly and hyperammonaemia after protein meals. Urine shows high excretion of lysine, arginine and ornithine, while plasma levels of these are low. How does the transport defect cause hyperammonaemia?

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

The answer and the reasoning

B. Depletion of ornithine and arginine limits urea cycle intermediates

Lysinuric protein intolerance is caused by defective transport of dibasic amino acids across basolateral membranes of intestine and renal tubules. Intracellular ornithine and arginine are depleted in hepatocytes, so the urea cycle cannot run and ammonia rises after protein loads. Citrulline supplementation helps.

Option by option

  • A. Failure of ammonia to cross the hepatocyte membrane

    Ammonia moves across membranes freely as NH3 and is not the transporter problem here.

  • B. Depletion of ornithine and arginine limits urea cycle intermediates (correct)

    Correct. Ornithine and arginine act as urea cycle carriers, and their depletion slows the cycle.

  • C. Blocked synthesis of N-acetylglutamate from glutamate

    NAGS synthesis is not impaired by this dibasic transport defect.

  • D. Direct inhibition of glutamine synthetase in the brain

    Glutamine synthetase inhibition is not part of this disorder.

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, Depletion of ornithine and arginine limits urea cycle intermediates. Lysinuric protein intolerance is caused by defective transport of dibasic amino acids across basolateral membranes of intestine and renal tubules.

Why is "Failure of ammonia to cross the hepatocyte membrane" not the answer?

Ammonia moves across membranes freely as NH3 and is not the transporter problem here.

Why is "Blocked synthesis of N-acetylglutamate from glutamate" not the answer?

NAGS synthesis is not impaired by this dibasic transport defect.

Why is "Direct inhibition of glutamine synthetase in the brain" not the answer?

Glutamine synthetase inhibition is not part of this disorder.

What concept does this question test?

It tests Amino acid metabolism: lysinuric protein intolerance in Biochemistry. Intracellular ornithine and arginine are depleted in hepatocytes, so the urea cycle cannot run and ammonia rises after protein loads. Citrulline supplementation helps.

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 Amino acid metabolism: lysinuric protein intolerance?

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.