Toxicology: salicylate poisoning: 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 · Toxicology: salicylate poisoning

Sample item

A 19-year-old woman is brought in 6 hours after an overdose of aspirin with tinnitus, tachypnoea and sweating. Arterial gas shows pH 7.46, PaCO2 22 mmHg and HCO3 15 mmol/L. Which interpretation and treatment are correct?

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

The answer and the reasoning

D. Mixed respiratory alkalosis and metabolic acidosis; intravenous sodium bicarbonate for urinary alkalinisation

Salicylate stimulates the respiratory centre, causing respiratory alkalosis, and uncouples oxidative phosphorylation, producing an anion-gap metabolic acidosis. Expected PaCO2 for HCO3 15 by Winter's formula is 1.5 x 15 + 8 = 30.5 (plus or minus 2), so an actual 22 is lower, confirming a coexisting respiratory alkalosis. Bicarbonate alkalinises urine and serum, trapping salicylate ionised and enhancing renal excretion, and haemodialysis is used for severe toxicity.

Option by option

  • A. Mixed metabolic alkalosis and respiratory acidosis; intubation and sedation

    The gas shows low HCO3 and low PaCO2, not metabolic alkalosis with respiratory acidosis, and sedation and intubation could worsen acidaemia.

  • B. Simple metabolic acidosis with appropriate compensation; intravenous sodium bicarbonate for pH correction only

    Compensation would give a PaCO2 near 30, not 22, so a simple metabolic acidosis is incorrect.

  • C. Pure respiratory alkalosis; rebreathing into a paper bag

    The raised pH is not due to respiratory alkalosis alone, as HCO3 is low, and paper bag rebreathing would not address the poison.

  • D. Mixed respiratory alkalosis and metabolic acidosis; intravenous sodium bicarbonate for urinary alkalinisation (correct)

    Correct. The low PaCO2 beyond expected compensation shows the mixed disorder, and bicarbonate enhances elimination.

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, Mixed respiratory alkalosis and metabolic acidosis; intravenous sodium bicarbonate for urinary alkalinisation. Salicylate stimulates the respiratory centre, causing respiratory alkalosis, and uncouples oxidative phosphorylation, producing an anion-gap metabolic acidosis.

Why is "Mixed metabolic alkalosis and respiratory acidosis; intubation and se…" not the answer?

The gas shows low HCO3 and low PaCO2, not metabolic alkalosis with respiratory acidosis, and sedation and intubation could worsen acidaemia.

Why is "Simple metabolic acidosis with appropriate compensation; intravenous…" not the answer?

Compensation would give a PaCO2 near 30, not 22, so a simple metabolic acidosis is incorrect.

Why is "Pure respiratory alkalosis; rebreathing into a paper bag" not the answer?

The raised pH is not due to respiratory alkalosis alone, as HCO3 is low, and paper bag rebreathing would not address the poison.

What concept does this question test?

It tests Toxicology: salicylate poisoning in General Medicine. Expected PaCO2 for HCO3 15 by Winter's formula is 1.5 x 15 + 8 = 30.5 (plus or minus 2), so an actual 22 is lower, confirming a coexisting respiratory alkalosis. Bicarbonate alkalinises urine and serum, trapping salicylate ionised and enhancing renal excretion, and haemodialysis is used for severe toxicity.

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 Toxicology: salicylate poisoning?

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.