Calcium homeostasis: ionised calcium: 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 · Calcium homeostasis: ionised calcium

Sample item

A 26-year-old woman who is hyperventilating has carpopedal spasm and perioral paraesthesia. Her total serum calcium is normal. What explains the tetany?

AHypocapnia reduces total body calciumBAlkalosis increases calcium binding to albumin and lowers ionised calciumCHyperventilation increases urinary calcium excretion acutelyDAlkalosis reduces albumin binding and raises ionised calcium

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

The answer and the reasoning

B. Alkalosis increases calcium binding to albumin and lowers ionised calcium

Alkalosis reduces hydrogen ion occupancy on albumin, exposing more negative sites that bind calcium. The ionised fraction, the physiologically active form, therefore falls even though total calcium is unchanged. The low ionised calcium increases neuromuscular excitability and causes tetany.

Option by option

A. Hypocapnia reduces total body calcium
Total body calcium is not altered over minutes by hypocapnia.
B. Alkalosis increases calcium binding to albumin and lowers ionised calcium (correct)
Correct. Greater albumin binding lowers ionised calcium in alkalosis.
C. Hyperventilation increases urinary calcium excretion acutely
No significant acute renal calcium loss occurs in hyperventilation.
D. Alkalosis reduces albumin binding and raises ionised calcium
Alkalosis increases rather than reduces binding, which would otherwise protect against tetany.

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 B, Alkalosis increases calcium binding to albumin and lowers ionised calcium. Alkalosis reduces hydrogen ion occupancy on albumin, exposing more negative sites that bind calcium.

Why is "Hypocapnia reduces total body calcium" not the answer?

Total body calcium is not altered over minutes by hypocapnia.

Why is "Hyperventilation increases urinary calcium excretion acutely" not the answer?

No significant acute renal calcium loss occurs in hyperventilation.

Why is "Alkalosis reduces albumin binding and raises ionised calcium" not the answer?

Alkalosis increases rather than reduces binding, which would otherwise protect against tetany.

What concept does this question test?

It tests Calcium homeostasis: ionised calcium in Physiology. The ionised fraction, the physiologically active form, therefore falls even though total calcium is unchanged. The low ionised calcium increases neuromuscular excitability and 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 Calcium homeostasis: ionised calcium?

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.