Toxicology: 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

Sample item

A jewellery worker is found dead with a smell of bitter almonds from the stomach contents and bright red post-mortem staining. What is the mechanism of the poison?

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

The answer and the reasoning

C. Inhibition of cytochrome c oxidase, blocking cellular respiration

Cyanide binds the ferric iron of cytochrome aa3 (cytochrome c oxidase), so tissues cannot use oxygen, venous blood stays oxygenated and the lividity looks bright red. Jewellers and electroplaters may have access to cyanide salts.

Option by option

  • A. Binding to haemoglobin to form carboxyhaemoglobin

    Carboxyhaemoglobin formation is the mechanism of carbon monoxide poisoning.

  • B. Inhibition of acetylcholinesterase

    Acetylcholinesterase inhibition is the mechanism of organophosphates, with a kerosene-like or garlic smell.

  • C. Inhibition of cytochrome c oxidase, blocking cellular respiration (correct)

    Correct. Blocking the terminal enzyme of the respiratory chain causes histotoxic hypoxia.

  • D. Oxidation of haemoglobin to methaemoglobin

    Methaemoglobin formation is caused by nitrites and aniline, and it produces chocolate-brown blood.

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 C, Inhibition of cytochrome c oxidase, blocking cellular respiration. Cyanide binds the ferric iron of cytochrome aa3 (cytochrome c oxidase), so tissues cannot use oxygen, venous blood stays oxygenated and the lividity looks bright red.

Why is "Binding to haemoglobin to form carboxyhaemoglobin" not the answer?

Carboxyhaemoglobin formation is the mechanism of carbon monoxide poisoning.

Why is "Inhibition of acetylcholinesterase" not the answer?

Acetylcholinesterase inhibition is the mechanism of organophosphates, with a kerosene-like or garlic smell.

Why is "Oxidation of haemoglobin to methaemoglobin" not the answer?

Methaemoglobin formation is caused by nitrites and aniline, and it produces chocolate-brown blood.

What concept does this question test?

It tests Toxicology in Forensic Medicine. Jewellers and electroplaters may have access to cyanide salts.

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?

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.