Toxicology: organophosphates: 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: organophosphates

Sample item

A farmer is brought in after drinking an organophosphate pesticide. He has pinpoint pupils, salivation, bronchorrhoea and fasciculations. Atropine is given. Which additional drug reactivates acetylcholinesterase, and why must it be started early?

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

The answer and the reasoning

B. Pralidoxime, which must be given before the enzyme-inhibitor complex 'ages' and becomes irreversible

Organophosphates phosphorylate acetylcholinesterase. Pralidoxime removes the phosphate group by nucleophilic attack and restores enzyme activity, especially at the neuromuscular junction, but after 'aging' (loss of an alkyl group) the bond cannot be broken. Atropine blocks the muscarinic effects but does not treat weakness.

Option by option

  • A. Naloxone, which reverses the nicotinic effects

    Naloxone treats opioid poisoning; pinpoint pupils here arise from cholinergic excess.

  • B. Pralidoxime, which must be given before the enzyme-inhibitor complex 'ages' and becomes irreversible (correct)

    Correct. Oximes work only before aging and help the nicotinic and muscle effects.

  • C. Physostigmine, which removes the phosphate group

    Physostigmine is itself a cholinesterase inhibitor and would worsen toxicity.

  • D. Neostigmine, which competes at the muscarinic receptor

    Neostigmine is an anticholinesterase and would add to toxicity.

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 Pharmacology questions

Questions about this Pharmacology question

What is the answer to this Pharmacology question?

The best answer is B, Pralidoxime, which must be given before the enzyme-inhibitor complex 'ages' and becomes irreversible. Organophosphates phosphorylate acetylcholinesterase.

Why is "Naloxone, which reverses the nicotinic effects" not the answer?

Naloxone treats opioid poisoning; pinpoint pupils here arise from cholinergic excess.

Why is "Physostigmine, which removes the phosphate group" not the answer?

Physostigmine is itself a cholinesterase inhibitor and would worsen toxicity.

Why is "Neostigmine, which competes at the muscarinic receptor" not the answer?

Neostigmine is an anticholinesterase and would add to toxicity.

What concept does this question test?

It tests Toxicology: organophosphates in Pharmacology. Pralidoxime removes the phosphate group by nucleophilic attack and restores enzyme activity, especially at the neuromuscular junction, but after 'aging' (loss of an alkyl group) the bond cannot be broken. Atropine blocks the muscarinic effects but does not treat weakness.

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: organophosphates?

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.