Acid-base: compensation: 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 · Acid-base: compensation

Sample item

An exhausted patient with severe diarrhoea has pH 7.10, bicarbonate 12 mmol/L and PaCO2 40 mmHg. Which interpretation is correct?

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

The answer and the reasoning

B. Metabolic acidosis with superimposed respiratory acidosis

Winter's formula gives an expected PaCO2 of 1.5 x 12 + 8 = 26 (range 24 to 28 mmHg). The measured value of 40 mmHg is well above this, so ventilation is failing to compensate, which means an additional respiratory acidosis. Pure respiratory acidosis would have a near-normal bicarbonate.

Option by option

  • A. Metabolic acidosis with superimposed respiratory alkalosis

    Respiratory alkalosis would need PaCO2 below 24 mmHg.

  • B. Metabolic acidosis with superimposed respiratory acidosis (correct)

    Correct. The PaCO2 is higher than the expected 26 mmHg.

  • C. Metabolic acidosis with appropriate respiratory compensation

    Appropriate compensation would show a PaCO2 near 26 mmHg.

  • D. Acute respiratory acidosis alone

    Acute respiratory acidosis alone would have a bicarbonate of about 24 to 27 mmol/L.

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 B, Metabolic acidosis with superimposed respiratory acidosis. Winter's formula gives an expected PaCO2 of 1.

Why is "Metabolic acidosis with superimposed respiratory alkalosis" not the answer?

Respiratory alkalosis would need PaCO2 below 24 mmHg.

Why is "Metabolic acidosis with appropriate respiratory compensation" not the answer?

Appropriate compensation would show a PaCO2 near 26 mmHg.

Why is "Acute respiratory acidosis alone" not the answer?

Acute respiratory acidosis alone would have a bicarbonate of about 24 to 27 mmol/L.

What concept does this question test?

It tests Acid-base: compensation in Physiology. 5 x 12 + 8 = 26 (range 24 to 28 mmHg). The measured value of 40 mmHg is well above this, so ventilation is failing to compensate, which means an additional respiratory acidosis. Pure respiratory acidosis would have a near-normal bicarbonate.

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 Acid-base: compensation?

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.