Amino acid metabolism: ammonia and the brain: 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: ammonia and the brain

Sample item

A man with decompensated cirrhosis has confusion and a flapping tremor with a blood ammonia level three times normal. Which process in the brain contributes most to the cerebral swelling in this condition?

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

The answer and the reasoning

B. Astrocytic glutamine accumulation from ammonia and glutamate, causing osmotic swelling

The brain has no urea cycle, so ammonia is detoxified in astrocytes by glutamine synthetase, which combines ammonia with glutamate in an ATP-requiring reaction. Glutamine is osmotically active and its accumulation causes astrocyte swelling and cerebral oedema, while consumption of glutamate and alpha-ketoglutarate disturbs neurotransmission and energy metabolism. Lactulose and rifaximin lower gut ammonia production.

Option by option

  • A. Conversion of ammonia to urea within astrocytes

    Astrocytes lack urea cycle enzymes, so urea cannot be made in the brain.

  • B. Astrocytic glutamine accumulation from ammonia and glutamate, causing osmotic swelling (correct)

    Correct. Glutamine is the osmotically active product of astrocytic ammonia detoxification.

  • C. Accumulation of alpha-ketoglutarate in neurons

    Alpha-ketoglutarate is depleted rather than accumulated because it is drawn off to make glutamate.

  • D. Overproduction of GABA from excess glutamine in the synapse

    GABA production is not the main driver of swelling, and the swelling arises from glutamine itself.

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, Astrocytic glutamine accumulation from ammonia and glutamate, causing osmotic swelling. The brain has no urea cycle, so ammonia is detoxified in astrocytes by glutamine synthetase, which combines ammonia with glutamate in an ATP-requiring reaction.

Why is "Conversion of ammonia to urea within astrocytes" not the answer?

Astrocytes lack urea cycle enzymes, so urea cannot be made in the brain.

Why is "Accumulation of alpha-ketoglutarate in neurons" not the answer?

Alpha-ketoglutarate is depleted rather than accumulated because it is drawn off to make glutamate.

Why is "Overproduction of GABA from excess glutamine in the synapse" not the answer?

GABA production is not the main driver of swelling, and the swelling arises from glutamine itself.

What concept does this question test?

It tests Amino acid metabolism: ammonia and the brain in Biochemistry. Glutamine is osmotically active and its accumulation causes astrocyte swelling and cerebral oedema, while consumption of glutamate and alpha-ketoglutarate disturbs neurotransmission and energy metabolism. Lactulose and rifaximin lower gut ammonia production.

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: ammonia and the brain?

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.