Nerve and muscle: hypercalcaemia 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: hypercalcaemia and excitability

Sample item

A patient with a calcium of 3.4 mmol/L from hyperparathyroidism is lethargic with hypotonia and depressed tendon reflexes. How does high extracellular calcium reduce neuromuscular excitability?

AIt depolarises the resting membrane potential toward thresholdBIt permanently inactivates the sodium-potassium ATPaseCIt raises the threshold for sodium channel opening by screening membrane surface chargeDIt blocks all acetylcholine release at the nerve terminal

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

The answer and the reasoning

C. It raises the threshold for sodium channel opening by screening membrane surface charge

Extracellular calcium binds the outer membrane surface and makes the field sensed by the voltage sensors more positive, so a larger depolarisation is needed to open sodium channels. High calcium therefore stabilises membranes and reduces excitability, whereas low calcium causes tetany.

Option by option

A. It depolarises the resting membrane potential toward threshold
Resting potential is little changed, and depolarisation toward threshold would increase excitability.
B. It permanently inactivates the sodium-potassium ATPase
Calcium does not directly inactivate the Na-K pump.
C. It raises the threshold for sodium channel opening by screening membrane surface charge (correct)
Correct. High calcium shifts the threshold so a stronger stimulus is needed.
D. It blocks all acetylcholine release at the nerve terminal
Calcium entry normally triggers release and does not abolish it.

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.

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is C, It raises the threshold for sodium channel opening by screening membrane surface charge. Extracellular calcium binds the outer membrane surface and makes the field sensed by the voltage sensors more positive, so a larger depolarisation is needed to open sodium channels.

Why is "It depolarises the resting membrane potential toward threshold" not the answer?

Resting potential is little changed, and depolarisation toward threshold would increase excitability.

Why is "It permanently inactivates the sodium-potassium ATPase" not the answer?

Calcium does not directly inactivate the Na-K pump.

Why is "It blocks all acetylcholine release at the nerve terminal" not the answer?

Calcium entry normally triggers release and does not abolish it.

What concept does this question test?

It tests Nerve and muscle: hypercalcaemia and excitability in Physiology. High calcium therefore stabilises membranes and reduces excitability, whereas low calcium causes tetany.

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: hypercalcaemia 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.