Toxicology: detection of phosphine: 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.

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Forensic Medicine · Toxicology: detection of phosphine

Sample item

In a patient suspected of aluminium phosphide ingestion, a bedside test is done on the gastric aspirate. A paper moistened with silver nitrate held over the aspirate turns black. What does this test detect?

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

The answer and the reasoning

A. Phosphine gas

Phosphine reduces silver nitrate to metallic silver, which blackens the paper. The test is a simple method for confirming aluminium phosphide poisoning. Organophosphates, cyanide and paraquat are tested for in different ways.

Option by option

  • A. Phosphine gas (correct)

    Correct. Phosphine blackens silver nitrate paper.

  • B. Organophosphate metabolites

    Organophosphates are identified by low cholinesterase activity, not by silver nitrate.

  • C. Free cyanide

    Cyanide is detected by the Prussian blue reaction and not by silver nitrate paper blackening.

  • D. Paraquat

    Paraquat is detected by the sodium dithionite urine test, which turns blue.

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 Forensic Medicine question

What is the answer to this Forensic Medicine question?

The best answer is A, Phosphine gas. Phosphine reduces silver nitrate to metallic silver, which blackens the paper.

Why is "Organophosphate metabolites" not the answer?

Organophosphates are identified by low cholinesterase activity, not by silver nitrate.

Why is "Free cyanide" not the answer?

Cyanide is detected by the Prussian blue reaction and not by silver nitrate paper blackening.

Why is "Paraquat" not the answer?

Paraquat is detected by the sodium dithionite urine test, which turns blue.

What concept does this question test?

It tests Toxicology: detection of phosphine in Forensic Medicine. The test is a simple method for confirming aluminium phosphide poisoning. Organophosphates, cyanide and paraquat are tested for in different ways.

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 Toxicology: detection of phosphine?

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.