Respiratory system: acclimatisation to altitude: 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 · Respiratory system: acclimatisation to altitude

Sample item

A trekker has spent a week at 4,000 metres. Compared with day one, her arterial blood gas shows a normal pH, a low PaCO2 and a low bicarbonate. Which process produced the normal pH?

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

The answer and the reasoning

D. Renal excretion of bicarbonate compensating for the respiratory alkalosis

Hypoxia stimulates ventilation through carotid bodies, which lowers PaCO2 and raises pH. Over days the kidney excretes bicarbonate and restores pH toward normal, which lets ventilation rise further.

Option by option

  • A. Hypoventilation raising PaCO2

    Hypoventilation would raise PaCO2, the opposite of the findings.

  • B. Increased renal ammonium excretion

    Ammonium excretion is part of the response to acidosis, not alkalosis.

  • C. Increased lactic acid production

    Lactate would give acidosis and a bicarbonate fall without hypocapnia.

  • D. Renal excretion of bicarbonate compensating for the respiratory alkalosis (correct)

    Correct. Renal bicarbonate loss compensates for hypocapnic alkalosis over several days.

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 D, Renal excretion of bicarbonate compensating for the respiratory alkalosis. Hypoxia stimulates ventilation through carotid bodies, which lowers PaCO2 and raises pH.

Why is "Hypoventilation raising PaCO2" not the answer?

Hypoventilation would raise PaCO2, the opposite of the findings.

Why is "Increased renal ammonium excretion" not the answer?

Ammonium excretion is part of the response to acidosis, not alkalosis.

Why is "Increased lactic acid production" not the answer?

Lactate would give acidosis and a bicarbonate fall without hypocapnia.

What concept does this question test?

It tests Respiratory system: acclimatisation to altitude in Physiology. Over days the kidney excretes bicarbonate and restores pH toward normal, which lets ventilation rise further.

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 Respiratory system: acclimatisation to altitude?

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.