General physiology: calcium and nerve 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 · General physiology: calcium and nerve excitability

Sample item

A 30-year-old woman develops perioral tingling and carpopedal spasm after total thyroidectomy. Serum calcium is 1.7 mmol/L. How does low extracellular calcium cause tetany?

AIt lowers the threshold for Na+ channel activation, so nerves discharge spontaneouslyBIt raises the threshold for Na+ channel activation, so nerves discharge less readilyCIt prevents troponin C from binding Ca2+ and so produces sustained contractionDIt blocks the release of acetylcholine, so muscle is left in a contracted state

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

The answer and the reasoning

A. It lowers the threshold for Na+ channel activation, so nerves discharge spontaneously

Extracellular Ca2+ binds the outer membrane surface and stabilises voltage-gated Na+ channels. When it falls, channels open at a less depolarised voltage, so peripheral nerves fire spontaneously and repetitively, giving paraesthesiae, Chvostek and Trousseau signs and tetany. Contraction itself still depends on intracellular Ca2+.

Option by option

A. It lowers the threshold for Na+ channel activation, so nerves discharge spontaneously (correct)
The lower activation threshold produces hyperexcitability and spontaneous discharge.
B. It raises the threshold for Na+ channel activation, so nerves discharge less readily
Hypercalcaemia raises the threshold, which causes weakness and hyporeflexia.
C. It prevents troponin C from binding Ca2+ and so produces sustained contraction
Troponin C binding depends on intracellular Ca2+ and is not blocked by a fall in extracellular Ca2+.
D. It blocks the release of acetylcholine, so muscle is left in a contracted state
Reduced acetylcholine release would cause weakness, not spasm.

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 A, It lowers the threshold for Na+ channel activation, so nerves discharge spontaneously. Extracellular Ca2+ binds the outer membrane surface and stabilises voltage-gated Na+ channels.

Why is "It raises the threshold for Na+ channel activation, so nerves dischar…" not the answer?

Hypercalcaemia raises the threshold, which causes weakness and hyporeflexia.

Why is "It prevents troponin C from binding Ca2+ and so produces sustained co…" not the answer?

Troponin C binding depends on intracellular Ca2+ and is not blocked by a fall in extracellular Ca2+.

Why is "It blocks the release of acetylcholine, so muscle is left in a contra…" not the answer?

Reduced acetylcholine release would cause weakness, not spasm.

What concept does this question test?

It tests General physiology: calcium and nerve excitability in Physiology. When it falls, channels open at a less depolarised voltage, so peripheral nerves fire spontaneously and repetitively, giving paraesthesiae, Chvostek and Trousseau signs and tetany. Contraction itself still depends on intracellular Ca2+.

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 General physiology: calcium and nerve 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.