Acid-base: compensation and mixed disorders: 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.

Try it

General Medicine · Acid-base: compensation and mixed disorders

Sample item

A septic patient is drowsy. Arterial blood gas shows pH 7.10, PaCO2 40 mmHg and bicarbonate 12 mmol/L. Sodium is 138 mmol/L and chloride 100 mmol/L. Using the expected compensation for metabolic acidosis (1.5 x bicarbonate + 8, plus or minus 2), which interpretation is correct?

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

The answer and the reasoning

D. High anion gap metabolic acidosis with concurrent respiratory acidosis

Anion gap is 138 minus 100 minus 12, which is 26, so the acidosis is of high anion gap type. Expected PaCO2 is 1.5 x 12 + 8 = 26, with a range of 24 to 28 mmHg. The measured 40 mmHg is well above this, so the lungs are failing to compensate, indicating coexisting respiratory acidosis from fatigue or depressed drive.

Option by option

  • A. High anion gap metabolic acidosis with appropriate compensation

    Appropriate compensation would give a PaCO2 of 24 to 28 mmHg, and 40 is higher.

  • B. Acute respiratory acidosis with renal compensation

    A PaCO2 of 40 is normal, so this cannot be respiratory acidosis alone, and the bicarbonate is far too low.

  • C. High anion gap metabolic acidosis with concurrent respiratory alkalosis

    Respiratory alkalosis would produce a PaCO2 below 24 mmHg.

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

    Correct. A PaCO2 of 40 exceeds the expected 24 to 28, which indicates inadequate ventilatory 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.

More General Medicine questions

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 concurrent respiratory acidosis. Anion gap is 138 minus 100 minus 12, which is 26, so the acidosis is of high anion gap type.

Why is "High anion gap metabolic acidosis with appropriate compensation" not the answer?

Appropriate compensation would give a PaCO2 of 24 to 28 mmHg, and 40 is higher.

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

A PaCO2 of 40 is normal, so this cannot be respiratory acidosis alone, and the bicarbonate is far too low.

Why is "High anion gap metabolic acidosis with concurrent respiratory alkalos…" not the answer?

Respiratory alkalosis would produce a PaCO2 below 24 mmHg.

What concept does this question test?

It tests Acid-base: compensation and mixed disorders in General Medicine. Expected PaCO2 is 1.5 x 12 + 8 = 26, with a range of 24 to 28 mmHg. The measured 40 mmHg is well above this, so the lungs are failing to compensate, indicating coexisting respiratory acidosis from fatigue or depressed drive.

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 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.