Toxicology: nitroprusside and cyanide: 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: nitroprusside and cyanide

Sample item

A 60-year-old man receives a high-dose sodium nitroprusside infusion for 3 days for hypertensive emergency. He develops confusion, tachypnoea and severe lactic acidosis with a high mixed venous oxygen saturation. Which agent is most appropriate?

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

The answer and the reasoning

D. Hydroxocobalamin

Nitroprusside releases cyanide, which binds cytochrome oxidase and prevents tissues from using oxygen, so venous blood stays oxygen-rich despite lactic acidosis. Hydroxocobalamin binds cyanide to form cyanocobalamin (vitamin B12), which is excreted in urine. Sodium thiosulfate, which converts cyanide to thiocyanate, can be added.

Option by option

  • A. Dimercaprol

    Dimercaprol chelates arsenic, mercury and gold.

  • B. Methylene blue

    Methylene blue treats methaemoglobinaemia and is not a cyanide binder.

  • C. Atropine

    Atropine treats cholinergic excess, which is not present.

  • D. Hydroxocobalamin (correct)

    Hydroxocobalamin binds cyanide to form cyanocobalamin.

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 D, Hydroxocobalamin. Nitroprusside releases cyanide, which binds cytochrome oxidase and prevents tissues from using oxygen, so venous blood stays oxygen-rich despite lactic acidosis.

Why is "Dimercaprol" not the answer?

Dimercaprol chelates arsenic, mercury and gold.

Why is "Methylene blue" not the answer?

Methylene blue treats methaemoglobinaemia and is not a cyanide binder.

Why is "Atropine" not the answer?

Atropine treats cholinergic excess, which is not present.

What concept does this question test?

It tests Toxicology: nitroprusside and cyanide in Pharmacology. Hydroxocobalamin binds cyanide to form cyanocobalamin (vitamin B12), which is excreted in urine. Sodium thiosulfate, which converts cyanide to thiocyanate, can be added.

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: nitroprusside and cyanide?

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.