Toxicology: methaemoglobinaemia: 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: methaemoglobinaemia

Sample item

A 30-year-old man taking dapsone develops cyanosis with SpO2 of 85% that does not improve on 100% oxygen. Arterial PaO2 is normal and his blood appears chocolate brown. What is the antidote?

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

The answer and the reasoning

D. Intravenous methylene blue

Oxidant drugs such as dapsone, nitrites and local anaesthetics convert haemoglobin iron to the ferric state, producing methaemoglobin that cannot carry oxygen. Features are cyanosis unresponsive to oxygen, a normal PaO2 and chocolate-brown blood. Methylene blue reduces methaemoglobin through NADPH-dependent methaemoglobin reductase, and is ineffective and hazardous in G6PD deficiency.

Option by option

  • A. Intravenous N-acetylcysteine

    N-acetylcysteine is the antidote for paracetamol poisoning.

  • B. Intravenous hydroxocobalamin

    Hydroxocobalamin is the antidote for cyanide poisoning.

  • C. Intravenous sodium nitrite

    Sodium nitrite creates methaemoglobin and would worsen this condition.

  • D. Intravenous methylene blue (correct)

    Correct. Methylene blue restores iron to the ferrous state so haemoglobin can carry oxygen again.

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 General Medicine question

What is the answer to this General Medicine question?

The best answer is D, Intravenous methylene blue. Oxidant drugs such as dapsone, nitrites and local anaesthetics convert haemoglobin iron to the ferric state, producing methaemoglobin that cannot carry oxygen.

Why is "Intravenous N-acetylcysteine" not the answer?

N-acetylcysteine is the antidote for paracetamol poisoning.

Why is "Intravenous hydroxocobalamin" not the answer?

Hydroxocobalamin is the antidote for cyanide poisoning.

Why is "Intravenous sodium nitrite" not the answer?

Sodium nitrite creates methaemoglobin and would worsen this condition.

What concept does this question test?

It tests Toxicology: methaemoglobinaemia in General Medicine. Features are cyanosis unresponsive to oxygen, a normal PaO2 and chocolate-brown blood. Methylene blue reduces methaemoglobin through NADPH-dependent methaemoglobin reductase, and is ineffective and hazardous in G6PD deficiency.

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: methaemoglobinaemia?

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.