Toxicology: carbon monoxide: 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: carbon monoxide

Sample item

A family in a poorly ventilated room warmed by a coal stove is found with headache, vomiting and drowsiness. Pulse oximetry reads 99% on room air. What is the most appropriate immediate treatment?

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

The answer and the reasoning

D. 100% oxygen through a non-rebreather mask

Carbon monoxide binds haemoglobin with about 200 times the affinity of oxygen, shifts the dissociation curve left and inhibits cytochrome oxidase. Standard pulse oximetry misreads carboxyhaemoglobin as oxyhaemoglobin, so the reading is falsely normal. High-flow 100% oxygen reduces the half-life of carboxyhaemoglobin from about 4 to 5 hours to about 1 hour, and hyperbaric oxygen is reserved for severe cases.

Option by option

  • A. Observation on room air until symptoms resolve

    Room air leaves the half-life prolonged and risks delayed neurological injury.

  • B. Intravenous sodium nitrite

    Sodium nitrite creates methaemoglobin and would reduce oxygen carrying capacity further.

  • C. Intravenous methylene blue

    Methylene blue treats methaemoglobinaemia and would not help.

  • D. 100% oxygen through a non-rebreather mask (correct)

    100% oxygen speeds displacement of carbon monoxide from haemoglobin.

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 Pharmacology question

What is the answer to this Pharmacology question?

The best answer is D, 100% oxygen through a non-rebreather mask. Carbon monoxide binds haemoglobin with about 200 times the affinity of oxygen, shifts the dissociation curve left and inhibits cytochrome oxidase.

Why is "Observation on room air until symptoms resolve" not the answer?

Room air leaves the half-life prolonged and risks delayed neurological injury.

Why is "Intravenous sodium nitrite" not the answer?

Sodium nitrite creates methaemoglobin and would reduce oxygen carrying capacity further.

Why is "Intravenous methylene blue" not the answer?

Methylene blue treats methaemoglobinaemia and would not help.

What concept does this question test?

It tests Toxicology: carbon monoxide in Pharmacology. Standard pulse oximetry misreads carboxyhaemoglobin as oxyhaemoglobin, so the reading is falsely normal. High-flow 100% oxygen reduces the half-life of carboxyhaemoglobin from about 4 to 5 hours to about 1 hour, and hyperbaric oxygen is reserved for severe cases.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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: carbon monoxide?

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.