Respiratory medicine: ABG interpretation: practice question

A General Medicine 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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General Medicine · Respiratory medicine: ABG interpretation

Sample item

ABG shows pH 7.30, PaCO2 25 mmHg, HCO3 12 mmol/L; Na 140, Cl 108. What is the acid-base disorder?

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

The answer and the reasoning

D. High anion gap metabolic acidosis with respiratory compensation

Low pH with low bicarbonate indicates metabolic acidosis, and the low PaCO2 is respiratory compensation. The anion gap is Na minus (Cl plus HCO3), which is 140 minus 120, giving 20 and well above the normal of about 12. Lactate, ketones, uraemia and toxins should be sought.

Option by option

  • A. Respiratory acidosis

    PaCO2 is low, which excludes respiratory acidosis.

  • B. Normal anion gap metabolic acidosis

    A gap of 20 is raised, so the gap is not normal.

  • C. Metabolic alkalosis

    pH and bicarbonate are low, which excludes alkalosis.

  • D. High anion gap metabolic acidosis with respiratory compensation (correct)

    Correct. The gap of 20 indicates high anion gap acidosis with 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.

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Questions about this General Medicine question

What is the answer to this General Medicine question?

The best answer is D, High anion gap metabolic acidosis with respiratory compensation. Low pH with low bicarbonate indicates metabolic acidosis, and the low PaCO2 is respiratory compensation.

Why is "Respiratory acidosis" not the answer?

PaCO2 is low, which excludes respiratory acidosis.

Why is "Normal anion gap metabolic acidosis" not the answer?

A gap of 20 is raised, so the gap is not normal.

Why is "Metabolic alkalosis" not the answer?

pH and bicarbonate are low, which excludes alkalosis.

What concept does this question test?

It tests Respiratory medicine: ABG interpretation in General Medicine. The anion gap is Na minus (Cl plus HCO3), which is 140 minus 120, giving 20 and well above the normal of about 12. Lactate, ketones, uraemia and toxins should be sought.

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 medicine: ABG interpretation?

For each disease know the classic presentation, the best test, and the first-line treatment. Then use the flashcards and the other General Medicine 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.