Antiparkinsonian drugs: COMT inhibitors: 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 · Antiparkinsonian drugs: COMT inhibitors

Sample item

A 62-year-old man on levodopa with carbidopa has motor fluctuations, with symptoms returning an hour before each next dose. Entacapone is added. How does it help?

AIt inhibits catechol-O-methyltransferase, reducing peripheral levodopa breakdown and prolonging its effectBIt stimulates D2 receptors directlyCIt inhibits monoamine oxidase B in the brainDIt blocks muscarinic receptors in the striatum

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

The answer and the reasoning

A. It inhibits catechol-O-methyltransferase, reducing peripheral levodopa breakdown and prolonging its effect

With peripheral dopa decarboxylase blocked by carbidopa, more levodopa is metabolised by COMT to 3-O-methyldopa. Entacapone blocks this, lengthening the plasma half-life of levodopa and smoothing end-of-dose wearing-off. Adverse effects include dyskinesia and brownish-orange urine.

Option by option

A. It inhibits catechol-O-methyltransferase, reducing peripheral levodopa breakdown and prolonging its effect (correct)
COMT inhibition extends levodopa duration and reduces wearing-off.
B. It stimulates D2 receptors directly
Direct D2 stimulation describes pramipexole and ropinirole.
C. It inhibits monoamine oxidase B in the brain
MAO-B inhibition describes selegiline and rasagiline.
D. It blocks muscarinic receptors in the striatum
Antimuscarinic action describes trihexyphenidyl.

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 Pharmacology question

What is the answer to this Pharmacology question?

The best answer is A, It inhibits catechol-O-methyltransferase, reducing peripheral levodopa breakdown and prolonging its effect. With peripheral dopa decarboxylase blocked by carbidopa, more levodopa is metabolised by COMT to 3-O-methyldopa.

Why is "It stimulates D2 receptors directly" not the answer?

Direct D2 stimulation describes pramipexole and ropinirole.

Why is "It inhibits monoamine oxidase B in the brain" not the answer?

MAO-B inhibition describes selegiline and rasagiline.

Why is "It blocks muscarinic receptors in the striatum" not the answer?

Antimuscarinic action describes trihexyphenidyl.

What concept does this question test?

It tests Antiparkinsonian drugs: COMT inhibitors in Pharmacology. Entacapone blocks this, lengthening the plasma half-life of levodopa and smoothing end-of-dose wearing-off. Adverse effects include dyskinesia and brownish-orange urine.

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 Antiparkinsonian drugs: COMT inhibitors?

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.