Clinical enzymology: plasma cholinesterase: practice question

A Biochemistry 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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Biochemistry · Clinical enzymology: plasma cholinesterase

Sample item

A 30-year-old man has 3 hours of apnoea after suxamethonium for an appendicectomy. His brother had a similar episode. Which statement best explains the event?

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

The answer and the reasoning

A. Atypical plasma cholinesterase hydrolyses suxamethonium slowly, and the dibucaine number is low

Suxamethonium is normally broken down quickly by plasma (butyrylcholine) esterase. An inherited atypical enzyme variant hydrolyses it slowly, causing prolonged paralysis. Dibucaine inhibits the normal enzyme by about 80% but the atypical enzyme much less, so a low dibucaine number identifies the defect, and treatment is ventilatory support until recovery.

Option by option

  • A. Atypical plasma cholinesterase hydrolyses suxamethonium slowly, and the dibucaine number is low (correct)

    Correct. An atypical enzyme variant with low dibucaine number causes prolonged apnoea.

  • B. Reduced acetylcholine receptors with a positive edrophonium test

    Myasthenia gravis would show a positive edrophonium response but not a family pattern of this type.

  • C. Ryanodine receptor mutation, with a positive caffeine-halothane contracture test

    A ryanodine receptor mutation causes malignant hyperthermia, with rigidity and fever, not isolated prolonged apnoea.

  • D. Extrajunctional receptors causing hyperkalaemic cardiac arrest

    Hyperkalaemia after suxamethonium occurs with burns or denervation and causes arrhythmia, not isolated prolonged apnoea.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, Atypical plasma cholinesterase hydrolyses suxamethonium slowly, and the dibucaine number is low. Suxamethonium is normally broken down quickly by plasma (butyrylcholine) esterase.

Why is "Reduced acetylcholine receptors with a positive edrophonium test" not the answer?

Myasthenia gravis would show a positive edrophonium response but not a family pattern of this type.

Why is "Ryanodine receptor mutation, with a positive caffeine-halothane contr…" not the answer?

A ryanodine receptor mutation causes malignant hyperthermia, with rigidity and fever, not isolated prolonged apnoea.

Why is "Extrajunctional receptors causing hyperkalaemic cardiac arrest" not the answer?

Hyperkalaemia after suxamethonium occurs with burns or denervation and causes arrhythmia, not isolated prolonged apnoea.

What concept does this question test?

It tests Clinical enzymology: plasma cholinesterase in Biochemistry. An inherited atypical enzyme variant hydrolyses it slowly, causing prolonged paralysis. Dibucaine inhibits the normal enzyme by about 80% but the atypical enzyme much less, so a low dibucaine number identifies the defect, and treatment is ventilatory support until recovery.

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 Clinical enzymology: plasma cholinesterase?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.