Acid-base: compensation and mixed disorders: 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 and mixed disorders

Sample item

A patient has pH 7.08, bicarbonate 10 mmol/L and PCO2 35 mmHg. Using Winter formula (expected PCO2 = 1.5 x HCO3 + 8, plus or minus 2), how should this be interpreted?

AMetabolic acidosis with appropriate respiratory compensationBMetabolic acidosis with a coexisting respiratory acidosisCMetabolic acidosis with a coexisting respiratory alkalosisDPrimary respiratory acidosis with renal compensation

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

The answer and the reasoning

B. Metabolic acidosis with a coexisting respiratory acidosis

Expected PCO2 = 1.5 x 10 + 8 = 23, with a range of 21 to 25. The measured value of 35 is above this range, so ventilation is inadequate for the degree of acidosis and a second respiratory acidosis is present. This might signal fatigue or depressed ventilation and warrants urgent support.

Option by option

A. Metabolic acidosis with appropriate respiratory compensation
Appropriate compensation would give a PCO2 of 21 to 25 mmHg.
B. Metabolic acidosis with a coexisting respiratory acidosis (correct)
Correct. Measured PCO2 is above the expected range, so the respiratory response is inadequate.
C. Metabolic acidosis with a coexisting respiratory alkalosis
Respiratory alkalosis would need a PCO2 below 21 mmHg.
D. Primary respiratory acidosis with renal compensation
A low bicarbonate with low pH points to a primary metabolic disorder, not renal compensation.

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.

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is B, Metabolic acidosis with a coexisting respiratory acidosis. Expected PCO2 = 1.

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

Appropriate compensation would give a PCO2 of 21 to 25 mmHg.

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

Respiratory alkalosis would need a PCO2 below 21 mmHg.

Why is "Primary respiratory acidosis with renal compensation" not the answer?

A low bicarbonate with low pH points to a primary metabolic disorder, not renal compensation.

What concept does this question test?

It tests Acid-base: compensation and mixed disorders in Physiology. 5 x 10 + 8 = 23, with a range of 21 to 25. The measured value of 35 is above this range, so ventilation is inadequate for the degree of acidosis and a second respiratory acidosis is present. This might signal fatigue or depressed ventilation and warrants urgent support.

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 and mixed disorders?

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.