Critical care: A-a gradient: practice question

A Anaesthesia 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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Anaesthesia · Critical care: A-a gradient

Sample item

A patient with an opioid overdose has a PaCO2 of 80 mmHg and hypoxaemia on room air. What is expected of the alveolar-arterial oxygen gradient?

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

The answer and the reasoning

C. Normal, because the hypoxaemia is due to hypoventilation

Pure hypoventilation raises alveolar CO2 and lowers alveolar and arterial oxygen together, so the difference between them stays normal. A raised gradient indicates V/Q mismatch, shunt or diffusion impairment. Correcting ventilation with naloxone or support restores oxygenation.

Option by option

  • A. Markedly raised from shunt

    Shunt widens the gradient and is not the mechanism here.

  • B. Markedly raised from diffusion defect

    A diffusion defect widens the gradient and is not the cause.

  • C. Normal, because the hypoxaemia is due to hypoventilation (correct)

    Correct. Hypoventilation lowers PaO2 without widening the gradient.

  • D. Negative

    A negative gradient is not physiologically possible.

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 Anaesthesia question

What is the answer to this Anaesthesia question?

The best answer is C, Normal, because the hypoxaemia is due to hypoventilation. Pure hypoventilation raises alveolar CO2 and lowers alveolar and arterial oxygen together, so the difference between them stays normal.

Why is "Markedly raised from shunt" not the answer?

Shunt widens the gradient and is not the mechanism here.

Why is "Markedly raised from diffusion defect" not the answer?

A diffusion defect widens the gradient and is not the cause.

Why is "Negative" not the answer?

A negative gradient is not physiologically possible.

What concept does this question test?

It tests Critical care: A-a gradient in Anaesthesia. A raised gradient indicates V/Q mismatch, shunt or diffusion impairment. Correcting ventilation with naloxone or support restores oxygenation.

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 Critical care: A-a gradient?

Make a table of each agent, its key advantage and its main side effect. Then use the flashcards and the other Anaesthesia 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.