General physiology: brain adaptation to chronic hyponatraemia: practice question

A Physiology 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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Physiology · General physiology: brain adaptation to chronic hyponatraemia

Sample item

A 45-year-old man with chronic alcohol use and serum sodium of 108 mmol/L for several days has it corrected to 125 mmol/L within 12 hours. Two days later he develops dysarthria and quadriparesis. Which physiological factor underlies this injury?

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

The answer and the reasoning

A. Brain cells had lost organic osmolytes, so a rapid rise in extracellular osmolality shrinks them

During chronic hyponatraemia, brain cells extrude organic osmolytes and ions to limit swelling. If sodium is then corrected too fast, the extracellular fluid becomes hypertonic relative to the adapted cells, which shrink, and myelin injury (osmotic demyelination) follows. Correction should therefore be gradual.

Option by option

  • A. Brain cells had lost organic osmolytes, so a rapid rise in extracellular osmolality shrinks them (correct)

    Correct. Adapted cells without osmolytes cannot tolerate a rapid osmolality rise.

  • B. Hypoglycaemia caused by the saline infusion

    Saline does not cause hypoglycaemia and the mechanism is osmotic.

  • C. Continued water entry into neurones causing acute cerebral oedema

    Swelling is the risk of acute hyponatraemia or overly rapid water gain, not of rapid correction.

  • D. Cerebral hypoperfusion from hypovolaemia

    Hypovolaemia is not the cause of the delayed demyelinating picture.

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 Physiology question

What is the answer to this Physiology question?

The best answer is A, Brain cells had lost organic osmolytes, so a rapid rise in extracellular osmolality shrinks them. During chronic hyponatraemia, brain cells extrude organic osmolytes and ions to limit swelling.

Why is "Hypoglycaemia caused by the saline infusion" not the answer?

Saline does not cause hypoglycaemia and the mechanism is osmotic.

Why is "Continued water entry into neurones causing acute cerebral oedema" not the answer?

Swelling is the risk of acute hyponatraemia or overly rapid water gain, not of rapid correction.

Why is "Cerebral hypoperfusion from hypovolaemia" not the answer?

Hypovolaemia is not the cause of the delayed demyelinating picture.

What concept does this question test?

It tests General physiology: brain adaptation to chronic hyponatraemia in Physiology. If sodium is then corrected too fast, the extracellular fluid becomes hypertonic relative to the adapted cells, which shrink, and myelin injury (osmotic demyelination) follows. Correction should therefore be gradual.

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 General physiology: brain adaptation to chronic hyponatraemia?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology 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.