Toxicology: cyanide antidote in smoke inhalation: practice question

A Forensic 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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Forensic Medicine · Toxicology: cyanide antidote in smoke inhalation

Sample item

A man rescued from a closed-room fire has soot around the mouth, shock, a lactate of 12 mmol/L and carboxyhaemoglobin of 25%. Co-existent cyanide poisoning is suspected. Which antidote is preferred?

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

The answer and the reasoning

A. Intravenous hydroxocobalamin

In smoke inhalation, hydroxocobalamin binds cyanide to form cyanocobalamin without reducing oxygen-carrying capacity. Sodium nitrite creates methaemoglobin to bind cyanide, which further reduces oxygen delivery in a patient who already has carboxyhaemoglobin and is therefore avoided.

Option by option

  • A. Intravenous hydroxocobalamin (correct)

    Correct. Hydroxocobalamin is safe in co-existing carbon monoxide exposure.

  • B. Intravenous sodium nitrite followed by thiosulphate

    Nitrites induce methaemoglobinaemia and worsen oxygen delivery.

  • C. Intramuscular pralidoxime

    Pralidoxime reactivates cholinesterase in organophosphate poisoning.

  • D. Intravenous methylene blue

    Methylene blue treats methaemoglobinaemia and is not a cyanide antidote.

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 Forensic Medicine question

What is the answer to this Forensic Medicine question?

The best answer is A, Intravenous hydroxocobalamin. In smoke inhalation, hydroxocobalamin binds cyanide to form cyanocobalamin without reducing oxygen-carrying capacity.

Why is "Intravenous sodium nitrite followed by thiosulphate" not the answer?

Nitrites induce methaemoglobinaemia and worsen oxygen delivery.

Why is "Intramuscular pralidoxime" not the answer?

Pralidoxime reactivates cholinesterase in organophosphate poisoning.

Why is "Intravenous methylene blue" not the answer?

Methylene blue treats methaemoglobinaemia and is not a cyanide antidote.

What concept does this question test?

It tests Toxicology: cyanide antidote in smoke inhalation in Forensic Medicine. Sodium nitrite creates methaemoglobin to bind cyanide, which further reduces oxygen delivery in a patient who already has carboxyhaemoglobin and is therefore avoided.

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 antidote in smoke inhalation?

Learn postmortem changes with their timelines, and toxicology as poison, sign, test and antidote. Then use the flashcards and the other Forensic 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.