Toxicology: aging of cholinesterase: 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: aging of cholinesterase

Sample item

Pralidoxime is most effective if given early after organophosphate poisoning. Why does delayed administration fail to reactivate the enzyme?

AThe organophosphate is excreted unchanged in urineBThe enzyme is quickly re-synthesised, so no oxime is neededCThe oxime is rapidly destroyed by plasma esterasesDAging: dealkylation makes the enzyme complex oxime-resistant

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

The answer and the reasoning

D. Aging: dealkylation makes the enzyme complex oxime-resistant

After organophosphate binding, the phosphorylated acetylcholinesterase undergoes aging, a loss of an alkyl group, which forms a stable negatively charged complex that oximes cannot reactivate. The rate depends on the compound, being faster for dimethyl than for diethyl agents. This is why oxime therapy is started as early as possible.

Option by option

A. The organophosphate is excreted unchanged in urine
Organophosphates are metabolised and bound to enzyme, not simply excreted unchanged.
B. The enzyme is quickly re-synthesised, so no oxime is needed
Enzyme re-synthesis is slow, and the oxime remains useful for as long as the enzyme is not aged.
C. The oxime is rapidly destroyed by plasma esterases
Oximes are not destroyed by plasma esterases in this way.
D. Aging: dealkylation makes the enzyme complex oxime-resistant (correct)
Correct. Aging creates an enzyme-inhibitor bond that oximes cannot break.

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.

Questions about this Forensic Medicine question

What is the answer to this Forensic Medicine question?

The best answer is D, Aging: dealkylation makes the enzyme complex oxime-resistant. After organophosphate binding, the phosphorylated acetylcholinesterase undergoes aging, a loss of an alkyl group, which forms a stable negatively charged complex that oximes cannot reactivate.

Why is "The organophosphate is excreted unchanged in urine" not the answer?

Organophosphates are metabolised and bound to enzyme, not simply excreted unchanged.

Why is "The enzyme is quickly re-synthesised, so no oxime is needed" not the answer?

Enzyme re-synthesis is slow, and the oxime remains useful for as long as the enzyme is not aged.

Why is "The oxime is rapidly destroyed by plasma esterases" not the answer?

Oximes are not destroyed by plasma esterases in this way.

What concept does this question test?

It tests Toxicology: aging of cholinesterase in Forensic Medicine. The rate depends on the compound, being faster for dimethyl than for diethyl agents. This is why oxime therapy is started as early as possible.

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: aging of cholinesterase?

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.