Toxicology: methaemoglobinaemia: practice question

A Forensic 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.

Try it

Forensic Medicine · Toxicology: methaemoglobinaemia

Sample item

A farm worker after exposure to a nitrite-containing chemical is cyanosed with oxygen saturation of 85% on pulse oximetry that does not improve with oxygen. His blood is chocolate brown. What is the treatment of choice?

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

The answer and the reasoning

D. Methylene blue

Nitrites oxidise haemoglobin iron to the ferric form, producing methaemoglobin that cannot carry oxygen, so cyanosis does not respond to oxygen. Methylene blue reduces methaemoglobin back to haemoglobin through NADPH-dependent reductase and is the antidote. It is ineffective and risky in G6PD deficiency, where alternatives such as exchange transfusion are used.

Option by option

  • A. Fomepizole

    Fomepizole treats ethylene glycol and methanol poisoning.

  • B. Hydroxocobalamin

    Hydroxocobalamin is the antidote for cyanide.

  • C. Atropine

    Atropine treats cholinergic poisoning.

  • D. Methylene blue (correct)

    Correct. Methylene blue restores iron to the ferrous state.

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.

More Forensic Medicine questions

Questions about this Forensic Medicine question

What is the answer to this Forensic Medicine question?

The best answer is D, Methylene blue. Nitrites oxidise haemoglobin iron to the ferric form, producing methaemoglobin that cannot carry oxygen, so cyanosis does not respond to oxygen.

Why is "Fomepizole" not the answer?

Fomepizole treats ethylene glycol and methanol poisoning.

Why is "Hydroxocobalamin" not the answer?

Hydroxocobalamin is the antidote for cyanide.

Why is "Atropine" not the answer?

Atropine treats cholinergic poisoning.

What concept does this question test?

It tests Toxicology: methaemoglobinaemia in Forensic Medicine. Methylene blue reduces methaemoglobin back to haemoglobin through NADPH-dependent reductase and is the antidote. It is ineffective and risky in G6PD deficiency, where alternatives such as exchange transfusion are used.

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 postmortem changes with their timelines, and toxicology as poison, sign, test and antidote. Then use the flashcards and the other Forensic 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.