Acid-base: compensation in metabolic acidosis: practice question

A Biochemistry 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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Biochemistry · Acid-base: compensation in metabolic acidosis

Sample item

A 20-year-old with diabetic ketoacidosis has been vomiting and is becoming drowsy. Arterial blood gas shows pH 7.10, bicarbonate 12 mmol/L and PaCO2 40 mmHg. Using Winter's formula, how should this be interpreted?

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

The answer and the reasoning

B. Superimposed respiratory acidosis, since expected PaCO2 is about 26 (24 to 28) mmHg

Winter's formula gives expected PaCO2 = 1.5 x HCO3 + 8 (plus or minus 2) = 1.5 x 12 + 8 = 26, so the range is 24 to 28 mmHg. A measured PaCO2 of 40 is far above this, meaning the respiratory system is failing to compensate and a respiratory acidosis is superimposed. This signals fatigue or depressed drive and is an indication for urgent airway and ventilatory assessment.

Option by option

  • A. Appropriate respiratory compensation for metabolic acidosis

    Appropriate compensation would give a PaCO2 close to 26 mmHg.

  • B. Superimposed respiratory acidosis, since expected PaCO2 is about 26 (24 to 28) mmHg (correct)

    Correct. Measured PaCO2 of 40 exceeds the expected 24 to 28, so ventilation is inadequate.

  • C. Superimposed metabolic alkalosis, since expected PaCO2 is about 52 mmHg

    The expected value is 26, not 52, and a PaCO2 above it does not indicate metabolic alkalosis.

  • D. Superimposed respiratory alkalosis, since expected PaCO2 is about 40 mmHg

    Respiratory alkalosis would produce a PaCO2 below the expected range, not above 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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Superimposed respiratory acidosis, since expected PaCO2 is about 26 (24 to 28) mmHg. Winter's formula gives expected PaCO2 = 1.

Why is "Appropriate respiratory compensation for metabolic acidosis" not the answer?

Appropriate compensation would give a PaCO2 close to 26 mmHg.

Why is "Superimposed metabolic alkalosis, since expected PaCO2 is about 52 mm…" not the answer?

The expected value is 26, not 52, and a PaCO2 above it does not indicate metabolic alkalosis.

Why is "Superimposed respiratory alkalosis, since expected PaCO2 is about 40…" not the answer?

Respiratory alkalosis would produce a PaCO2 below the expected range, not above it.

What concept does this question test?

It tests Acid-base: compensation in metabolic acidosis in Biochemistry. 5 x HCO3 + 8 (plus or minus 2) = 1.5 x 12 + 8 = 26, so the range is 24 to 28 mmHg. A measured PaCO2 of 40 is far above this, meaning the respiratory system is failing to compensate and a respiratory acidosis is superimposed. This signals fatigue or depressed drive and is an indication for urgent airway and ventilatory assessment.

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 in metabolic acidosis?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.