Parkinsonism: vitamin B6 and levodopa: 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 · Parkinsonism: vitamin B6 and levodopa

Sample item

A patient receiving levodopa alone takes a multivitamin containing pyridoxine and his symptom control worsens. What explains this?

APyridoxine inhibits COMTBPyridoxine is a cofactor that enhances peripheral dopa decarboxylase, converting levodopa to dopamine outside the brainCPyridoxine inhibits levodopa absorption by chelationDPyridoxine blocks dopamine receptors

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

The answer and the reasoning

B. Pyridoxine is a cofactor that enhances peripheral dopa decarboxylase, converting levodopa to dopamine outside the brain

Vitamin B6 is a cofactor of dopa decarboxylase, so in the absence of carbidopa more levodopa is decarboxylated peripherally and less reaches the brain. Carbidopa avoids this problem, so the interaction is not seen with levodopa-carbidopa.

Option by option

A. Pyridoxine inhibits COMT
COMT inhibition would increase levodopa effect.
B. Pyridoxine is a cofactor that enhances peripheral dopa decarboxylase, converting levodopa to dopamine outside the brain (correct)
Correct. Peripheral decarboxylation reduces central levodopa.
C. Pyridoxine inhibits levodopa absorption by chelation
Chelation is not the mechanism.
D. Pyridoxine blocks dopamine receptors
Pyridoxine does not block dopamine receptors.

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 B, Pyridoxine is a cofactor that enhances peripheral dopa decarboxylase, converting levodopa to dopamine outside the brain. Vitamin B6 is a cofactor of dopa decarboxylase, so in the absence of carbidopa more levodopa is decarboxylated peripherally and less reaches the brain.

Why is "Pyridoxine inhibits COMT" not the answer?

COMT inhibition would increase levodopa effect.

Why is "Pyridoxine inhibits levodopa absorption by chelation" not the answer?

Chelation is not the mechanism.

Why is "Pyridoxine blocks dopamine receptors" not the answer?

Pyridoxine does not block dopamine receptors.

What concept does this question test?

It tests Parkinsonism: vitamin B6 and levodopa in Pharmacology. Carbidopa avoids this problem, so the interaction is not seen with levodopa-carbidopa.

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 Parkinsonism: vitamin B6 and levodopa?

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.