Toxicology and antidotes: smoke inhalation and cyanide: practice question

A Pharmacology 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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Pharmacology · Toxicology and antidotes: smoke inhalation and cyanide

Sample item

A man rescued from an enclosed house fire is comatose and hypotensive with a lactate of 12 mmol/L and a raised carboxyhaemoglobin. Cyanide toxicity is suspected. Which antidote is preferred and why?

AFomepizole, because it blocks cyanide productionBHydroxocobalamin, because it does not reduce oxygen-carrying capacityCMethylene blue, because it reverses carboxyhaemoglobinDSodium nitrite, because it generates methaemoglobin to bind cyanide

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

The answer and the reasoning

B. Hydroxocobalamin, because it does not reduce oxygen-carrying capacity

Smoke inhalation often combines carbon monoxide and cyanide. Sodium nitrite creates methaemoglobin, which further reduces oxygen delivery in a patient who already has carboxyhaemoglobin, and it also causes hypotension. Hydroxocobalamin binds cyanide to form cyanocobalamin without these effects.

Option by option

A. Fomepizole, because it blocks cyanide production
Fomepizole inhibits alcohol dehydrogenase and is used for toxic alcohol poisoning.
B. Hydroxocobalamin, because it does not reduce oxygen-carrying capacity (correct)
Correct. Hydroxocobalamin binds cyanide without adding methaemoglobin or hypotension.
C. Methylene blue, because it reverses carboxyhaemoglobin
Methylene blue treats methaemoglobinaemia and does not remove carbon monoxide.
D. Sodium nitrite, because it generates methaemoglobin to bind cyanide
Nitrite worsens oxygen carriage by forming methaemoglobin and can lower blood pressure.

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 Pharmacology question

What is the answer to this Pharmacology question?

The best answer is B, Hydroxocobalamin, because it does not reduce oxygen-carrying capacity. Smoke inhalation often combines carbon monoxide and cyanide.

Why is "Fomepizole, because it blocks cyanide production" not the answer?

Fomepizole inhibits alcohol dehydrogenase and is used for toxic alcohol poisoning.

Why is "Methylene blue, because it reverses carboxyhaemoglobin" not the answer?

Methylene blue treats methaemoglobinaemia and does not remove carbon monoxide.

Why is "Sodium nitrite, because it generates methaemoglobin to bind cyanide" not the answer?

Nitrite worsens oxygen carriage by forming methaemoglobin and can lower blood pressure.

What concept does this question test?

It tests Toxicology and antidotes: smoke inhalation and cyanide in Pharmacology. Sodium nitrite creates methaemoglobin, which further reduces oxygen delivery in a patient who already has carboxyhaemoglobin, and it also causes hypotension. Hydroxocobalamin binds cyanide to form cyanocobalamin without these effects.

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 and antidotes: smoke inhalation and cyanide?

Learn drugs as classes, then the exception in each class that an examiner would pick. Then use the flashcards and the other Pharmacology 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.