Respiration: 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 · Respiration

Sample item

The central chemoreceptors in the medulla respond mainly to a change in which of the following?

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

The answer and the reasoning

C. Hydrogen ion concentration in the brain extracellular fluid

CO₂ crosses the blood–brain barrier and forms H⁺ in the CSF and brain interstitium. This H⁺ drives the central chemoreceptors. Peripheral chemoreceptors (carotid and aortic bodies) are the ones that respond to a low PO₂.

Option by option

  • A. Arterial bicarbonate rising

    Bicarbonate does not cross the barrier quickly, so it is not the stimulus.

  • B. Arterial PO₂

    A low PaO₂ is sensed by the peripheral chemoreceptors, not the central ones.

  • C. Hydrogen ion concentration in the brain extracellular fluid (correct)

    Correct. Central chemoreceptors read CO₂ indirectly through H⁺.

  • D. Arterial PCO₂ falling

    A fall in PaCO₂ would reduce ventilation, not stimulate 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.

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Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is C, Hydrogen ion concentration in the brain extracellular fluid. CO₂ crosses the blood–brain barrier and forms H⁺ in the CSF and brain interstitium.

Why is "Arterial bicarbonate rising" not the answer?

Bicarbonate does not cross the barrier quickly, so it is not the stimulus.

Why is "Arterial PO₂" not the answer?

A low PaO₂ is sensed by the peripheral chemoreceptors, not the central ones.

Why is "Arterial PCO₂ falling" not the answer?

A fall in PaCO₂ would reduce ventilation, not stimulate it.

What concept does this question test?

It tests Respiration in Physiology. This H⁺ drives the central chemoreceptors. Peripheral chemoreceptors (carotid and aortic bodies) are the ones that respond to a low PO₂.

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 Respiration?

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.