Leprosy: dapsone-induced methaemoglobinaemia: practice question

A Dermatology 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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Dermatology · Leprosy: dapsone-induced methaemoglobinaemia

Sample item

A leprosy patient on dapsone becomes cyanosed. His blood is chocolate brown, pulse oximetry reads about 85 percent, and arterial oxygen tension is normal. Which antidote is given?

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

The answer and the reasoning

C. Methylene blue

Dapsone oxidises haemoglobin to methaemoglobin, causing cyanosis with a normal arterial oxygen tension and a fixed low oximeter reading. Methylene blue reduces methaemoglobin back to haemoglobin, but it should be used cautiously in G6PD deficiency.

Option by option

  • A. N-acetylcysteine

    N-acetylcysteine is the antidote for paracetamol overdose.

  • B. Hydroxocobalamin

    Hydroxocobalamin is the antidote for cyanide poisoning.

  • C. Methylene blue (correct)

    Correct. Methylene blue is the antidote for symptomatic methaemoglobinaemia.

  • D. Flumazenil

    Flumazenil reverses benzodiazepine sedation.

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 Dermatology question

What is the answer to this Dermatology question?

The best answer is C, Methylene blue. Dapsone oxidises haemoglobin to methaemoglobin, causing cyanosis with a normal arterial oxygen tension and a fixed low oximeter reading.

Why is "N-acetylcysteine" not the answer?

N-acetylcysteine is the antidote for paracetamol overdose.

Why is "Hydroxocobalamin" not the answer?

Hydroxocobalamin is the antidote for cyanide poisoning.

Why is "Flumazenil" not the answer?

Flumazenil reverses benzodiazepine sedation.

What concept does this question test?

It tests Leprosy: dapsone-induced methaemoglobinaemia in Dermatology. Methylene blue reduces methaemoglobin back to haemoglobin, but it should be used cautiously in G6PD deficiency.

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 Leprosy: dapsone-induced methaemoglobinaemia?

Learn each condition by its primary lesion, site and signature sign. Then use the flashcards and the other Dermatology 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.