Clinical biochemistry: ionised calcium in hyperventilation: practice question

A Biochemistry 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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Biochemistry · Clinical biochemistry: ionised calcium in hyperventilation

Sample item

A 22-year-old with a panic attack has rapid breathing, perioral tingling and carpopedal spasm. Serum total calcium is normal. What explains the tetany?

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

The answer and the reasoning

B. Alkalosis increases calcium binding to albumin, lowering the ionised fraction

Alkalosis makes albumin more negatively charged, so it binds more calcium and the physiologically active ionised fraction falls even though total calcium is unchanged. Low ionised calcium increases neuromuscular excitability, causing tingling and carpopedal spasm. Rebreathing into a bag corrects the alkalosis.

Option by option

  • A. Hyperventilation causes renal calcium loss, lowering the ionised fraction

    Total calcium is normal, so there is no renal loss.

  • B. Alkalosis increases calcium binding to albumin, lowering the ionised fraction (correct)

    Correct. Alkalosis shifts calcium onto albumin and lowers ionised calcium.

  • C. Respiratory alkalosis suppresses PTH secretion, lowering ionised calcium

    Acute alkalosis does not suppress PTH enough to explain tetany.

  • D. Hyperventilation lowers magnesium, which blocks neuromuscular transmission

    Hyperventilation does not lower magnesium.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Alkalosis increases calcium binding to albumin, lowering the ionised fraction. Alkalosis makes albumin more negatively charged, so it binds more calcium and the physiologically active ionised fraction falls even though total calcium is unchanged.

Why is "Hyperventilation causes renal calcium loss, lowering the ionised frac…" not the answer?

Total calcium is normal, so there is no renal loss.

Why is "Respiratory alkalosis suppresses PTH secretion, lowering ionised calc…" not the answer?

Acute alkalosis does not suppress PTH enough to explain tetany.

Why is "Hyperventilation lowers magnesium, which blocks neuromuscular transmi…" not the answer?

Hyperventilation does not lower magnesium.

What concept does this question test?

It tests Clinical biochemistry: ionised calcium in hyperventilation in Biochemistry. Low ionised calcium increases neuromuscular excitability, causing tingling and carpopedal spasm. Rebreathing into a bag corrects the alkalosis.

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 Clinical biochemistry: ionised calcium in hyperventilation?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.