Amino acid metabolism: hepatic encephalopathy: 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: hepatic encephalopathy

Sample item

A 52-year-old man with alcoholic cirrhosis becomes drowsy with asterixis after a gastrointestinal bleed. His plasma ammonia is raised. Which process in the brain contributes most directly to cerebral oedema in this condition?

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

The answer and the reasoning

C. Astrocytic glutamine synthesis from ammonia and glutamate causing osmotic swelling

The brain lacks a urea cycle, so astrocytes detoxify ammonia by glutamine synthetase, forming glutamine from glutamate. Glutamine accumulation is osmotically active and causes astrocyte swelling, and also depletes alpha-ketoglutarate, which impairs energy metabolism. Treatment lowers gut ammonia production with lactulose and rifaximin.

Option by option

  • A. Accumulation of alpha-ketoglutarate drawing water into astrocytes

    Ammonia detoxification in astrocytes consumes alpha-ketoglutarate, so it is depleted rather than accumulated.

  • B. Depletion of inhibitory GABA from synapses

    GABAergic tone is enhanced in hepatic encephalopathy rather than depleted.

  • C. Astrocytic glutamine synthesis from ammonia and glutamate causing osmotic swelling (correct)

    Correct. Glutamine accumulation within astrocytes draws water and swells the cells.

  • D. Neuronal accumulation of urea crossing the blood-brain barrier

    Urea is formed in the liver and uraemic effects occur in renal failure, not here.

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 C, Astrocytic glutamine synthesis from ammonia and glutamate causing osmotic swelling. The brain lacks a urea cycle, so astrocytes detoxify ammonia by glutamine synthetase, forming glutamine from glutamate.

Why is "Accumulation of alpha-ketoglutarate drawing water into astrocytes" not the answer?

Ammonia detoxification in astrocytes consumes alpha-ketoglutarate, so it is depleted rather than accumulated.

Why is "Depletion of inhibitory GABA from synapses" not the answer?

GABAergic tone is enhanced in hepatic encephalopathy rather than depleted.

Why is "Neuronal accumulation of urea crossing the blood-brain barrier" not the answer?

Urea is formed in the liver and uraemic effects occur in renal failure, not here.

What concept does this question test?

It tests Amino acid metabolism: hepatic encephalopathy in Biochemistry. Glutamine accumulation is osmotically active and causes astrocyte swelling, and also depletes alpha-ketoglutarate, which impairs energy metabolism. Treatment lowers gut ammonia production with lactulose and rifaximin.

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: hepatic encephalopathy?

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.