Nerve and muscle: hyperkalaemia and excitability: 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 · Nerve and muscle: hyperkalaemia and excitability

Sample item

A patient with a serum potassium of 7.5 mmol/L has generalised muscle weakness. Why does persistently raised extracellular potassium reduce the excitability of skeletal muscle?

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

The answer and the reasoning

A. Sustained depolarisation inactivates sodium channels

Raised extracellular potassium shifts the potassium equilibrium potential and depolarises the resting membrane. At first this brings the cell closer to threshold, but with persistence sodium channels stay in the inactivated state, so no action potentials can fire and weakness results.

Option by option

  • A. Sustained depolarisation inactivates sodium channels (correct)

    Correct. Prolonged depolarisation inactivates sodium channels and blocks firing.

  • B. It prevents calcium release from the sarcoplasmic reticulum

    Calcium release depends on a muscle action potential and is not directly prevented by potassium.

  • C. It blocks acetylcholine receptors at the end plate

    Potassium does not block the end plate acetylcholine receptors.

  • D. It hyperpolarises the muscle membrane away from threshold

    Hyperkalaemia depolarises rather than hyperpolarises the membrane.

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, Sustained depolarisation inactivates sodium channels. Raised extracellular potassium shifts the potassium equilibrium potential and depolarises the resting membrane.

Why is "It prevents calcium release from the sarcoplasmic reticulum" not the answer?

Calcium release depends on a muscle action potential and is not directly prevented by potassium.

Why is "It blocks acetylcholine receptors at the end plate" not the answer?

Potassium does not block the end plate acetylcholine receptors.

Why is "It hyperpolarises the muscle membrane away from threshold" not the answer?

Hyperkalaemia depolarises rather than hyperpolarises the membrane.

What concept does this question test?

It tests Nerve and muscle: hyperkalaemia and excitability in Physiology. At first this brings the cell closer to threshold, but with persistence sodium channels stay in the inactivated state, so no action potentials can fire and weakness results.

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 Nerve and muscle: hyperkalaemia and excitability?

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.