Toxicology: methaemoglobinaemia: 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: methaemoglobinaemia

Sample item

A young man treated with dapsone becomes cyanosed with a saturation of 85% that does not improve on oxygen. His arterial blood is chocolate brown and PaO2 is normal. What is the treatment of choice?

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

The answer and the reasoning

A. Intravenous methylene blue

Dapsone, nitrites, benzocaine and primaquine oxidise haemoglobin iron to the ferric state, which cannot carry oxygen. Methylene blue, at 1 to 2 mg/kg, is reduced to leucomethylene blue by NADPH-dependent methaemoglobin reductase and converts methaemoglobin back to haemoglobin. It is ineffective and can cause haemolysis in G6PD deficiency, in which case exchange transfusion or ascorbic acid is considered.

Option by option

  • A. Intravenous methylene blue (correct)

    Methylene blue accelerates reduction of methaemoglobin to haemoglobin.

  • B. Hydroxocobalamin

    Hydroxocobalamin is a cyanide antidote and is not used for this.

  • C. Intravenous N-acetylcysteine

    N-acetylcysteine is the paracetamol antidote.

  • D. Hyperbaric oxygen

    Hyperbaric oxygen is used for carbon monoxide poisoning and does not correct ferric 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 A, Intravenous methylene blue. Dapsone, nitrites, benzocaine and primaquine oxidise haemoglobin iron to the ferric state, which cannot carry oxygen.

Why is "Hydroxocobalamin" not the answer?

Hydroxocobalamin is a cyanide antidote and is not used for this.

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

N-acetylcysteine is the paracetamol antidote.

Why is "Hyperbaric oxygen" not the answer?

Hyperbaric oxygen is used for carbon monoxide poisoning and does not correct ferric haemoglobin.

What concept does this question test?

It tests Toxicology: methaemoglobinaemia in Pharmacology. Methylene blue, at 1 to 2 mg/kg, is reduced to leucomethylene blue by NADPH-dependent methaemoglobin reductase and converts methaemoglobin back to haemoglobin. It is ineffective and can cause haemolysis in G6PD deficiency, in which case exchange transfusion or ascorbic acid is considered.

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?

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.