Toxicology: carbon monoxide 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: carbon monoxide poisoning

Sample item

A family is found drowsy in a closed room where a coal stove was burning overnight. They have headache and vomiting, and pulse oximetry reads 98% on room air. What is the most appropriate management?

A100% oxygen by non-rebreather mask, with carboxyhaemoglobin measured by co-oximetryBIntravenous methylene blueCIntravenous hydroxocobalaminDReassurance because the oxygen saturation is normal

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

The answer and the reasoning

A. 100% oxygen by non-rebreather mask, with carboxyhaemoglobin measured by co-oximetry

Carbon monoxide binds haemoglobin with about 200 times the affinity of oxygen and shifts the dissociation curve left. Standard pulse oximeters cannot distinguish carboxyhaemoglobin from oxyhaemoglobin and read falsely normal. Treatment is 100% oxygen, which shortens the half-life of carboxyhaemoglobin from about 5 hours to about 1 hour, with hyperbaric oxygen considered in severe cases.

Option by option

A. 100% oxygen by non-rebreather mask, with carboxyhaemoglobin measured by co-oximetry (correct)
Correct. High-flow oxygen displaces carbon monoxide, and co-oximetry confirms the diagnosis.
B. Intravenous methylene blue
Methylene blue treats methaemoglobinaemia and is not an antidote for carbon monoxide.
C. Intravenous hydroxocobalamin
Hydroxocobalamin treats cyanide poisoning, which is not the problem here.
D. Reassurance because the oxygen saturation is normal
A normal reading is misleading in this condition and cannot exclude 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, 100% oxygen by non-rebreather mask, with carboxyhaemoglobin measured by co-oximetry. Carbon monoxide binds haemoglobin with about 200 times the affinity of oxygen and shifts the dissociation curve left.

Why is "Intravenous methylene blue" not the answer?

Methylene blue treats methaemoglobinaemia and is not an antidote for carbon monoxide.

Why is "Intravenous hydroxocobalamin" not the answer?

Hydroxocobalamin treats cyanide poisoning, which is not the problem here.

Why is "Reassurance because the oxygen saturation is normal" not the answer?

A normal reading is misleading in this condition and cannot exclude poisoning.

What concept does this question test?

It tests Toxicology: carbon monoxide poisoning in General Medicine. Standard pulse oximeters cannot distinguish carboxyhaemoglobin from oxyhaemoglobin and read falsely normal. Treatment is 100% oxygen, which shortens the half-life of carboxyhaemoglobin from about 5 hours to about 1 hour, with hyperbaric oxygen considered in 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 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.