Clinical biochemistry: mixed acid-base disorder in salicylate poisoning: 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 · Clinical biochemistry: mixed acid-base disorder in salicylate poisoning

Sample item

A 20-year-old is brought after ingesting a large number of analgesic tablets and has tinnitus, fever and rapid deep breathing. ABG shows pH 7.46, PaCO2 22 mmHg and HCO3 15 mmol/L; Na is 140 and Cl 100 mmol/L. What is the acid-base disorder?

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

The answer and the reasoning

A. Respiratory alkalosis with a high anion gap metabolic acidosis

The anion gap is 140 minus 100 minus 15, which is 25, so there is a high anion gap acidosis. The expected PaCO2 from Winter formula is about 1.5 times 15 plus 8, or about 30, but the measured value is 22, so a primary respiratory alkalosis is also present. Salicylate stimulates the respiratory centre and uncouples oxidative phosphorylation.

Option by option

  • A. Respiratory alkalosis with a high anion gap metabolic acidosis (correct)

    Correct. A raised gap with PaCO2 lower than expected indicates both disorders.

  • B. Pure respiratory alkalosis with renal compensation only

    A pure respiratory alkalosis would not have a raised anion gap and low bicarbonate this large.

  • C. Metabolic alkalosis with a respiratory acidosis

    Metabolic alkalosis with respiratory acidosis would give a raised bicarbonate and PaCO2.

  • D. Pure high anion gap metabolic acidosis with full respiratory compensation

    With full compensation PaCO2 would be near 30, not 22, so an extra respiratory alkalosis exists.

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 A, Respiratory alkalosis with a high anion gap metabolic acidosis. The anion gap is 140 minus 100 minus 15, which is 25, so there is a high anion gap acidosis.

Why is "Pure respiratory alkalosis with renal compensation only" not the answer?

A pure respiratory alkalosis would not have a raised anion gap and low bicarbonate this large.

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

Metabolic alkalosis with respiratory acidosis would give a raised bicarbonate and PaCO2.

Why is "Pure high anion gap metabolic acidosis with full respiratory compensa…" not the answer?

With full compensation PaCO2 would be near 30, not 22, so an extra respiratory alkalosis exists.

What concept does this question test?

It tests Clinical biochemistry: mixed acid-base disorder in salicylate poisoning in Biochemistry. The expected PaCO2 from Winter formula is about 1.5 times 15 plus 8, or about 30, but the measured value is 22, so a primary respiratory alkalosis is also present. Salicylate stimulates the respiratory centre and uncouples oxidative phosphorylation.

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 Clinical biochemistry: mixed acid-base disorder in salicylate poisoning?

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.