Toxicology: cyanide 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: cyanide poisoning

Sample item

A 30-year-old man is rescued from an enclosed house fire with soot around the nostrils. He is unresponsive (GCS 8), hypotensive, and arterial lactate is 12 mmol/L. Pulse oximetry reads 99% but co-oximetry also suggests some carboxyhaemoglobin. Which antidote is most appropriate?

AHydroxocobalaminBSodium nitriteCAtropineDMethylene blue

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

The answer and the reasoning

A. Hydroxocobalamin

Burning of plastics and wool releases hydrogen cyanide, which inhibits cytochrome c oxidase and halts aerobic respiration, giving profound lactic acidosis despite adequate oxygen saturation. Hydroxocobalamin binds cyanide to form cyanocobalamin and is preferred in smoke inhalation. Nitrites generate methaemoglobin, which reduces oxygen delivery further in a patient who may also have carbon monoxide poisoning.

Option by option

A. Hydroxocobalamin (correct)
Correct. Hydroxocobalamin safely binds cyanide without impairing oxygen transport.
B. Sodium nitrite
Sodium nitrite induces methaemoglobinaemia and worsens oxygen delivery in smoke inhalation with carbon monoxide.
C. Atropine
Atropine treats cholinergic toxicity such as organophosphate poisoning.
D. Methylene blue
Methylene blue treats methaemoglobinaemia and has no role in cyanide poisoning.

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.

Questions about this General Medicine question

What is the answer to this General Medicine question?

The best answer is A, Hydroxocobalamin. Burning of plastics and wool releases hydrogen cyanide, which inhibits cytochrome c oxidase and halts aerobic respiration, giving profound lactic acidosis despite adequate oxygen saturation.

Why is "Sodium nitrite" not the answer?

Sodium nitrite induces methaemoglobinaemia and worsens oxygen delivery in smoke inhalation with carbon monoxide.

Why is "Atropine" not the answer?

Atropine treats cholinergic toxicity such as organophosphate poisoning.

Why is "Methylene blue" not the answer?

Methylene blue treats methaemoglobinaemia and has no role in cyanide poisoning.

What concept does this question test?

It tests Toxicology: cyanide poisoning in General Medicine. Hydroxocobalamin binds cyanide to form cyanocobalamin and is preferred in smoke inhalation. Nitrites generate methaemoglobin, which reduces oxygen delivery further in a patient who may also have carbon monoxide poisoning.

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: cyanide 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.