Amino acid metabolism: levodopa with carbidopa: 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.

Try it

Biochemistry · Amino acid metabolism: levodopa with carbidopa

Sample item

A patient with Parkinson disease receives levodopa combined with carbidopa. Carbidopa does not cross the blood-brain barrier. What is the biochemical purpose of adding it?

AIt blocks peripheral conversion of levodopa to dopamineBIt converts dopamine to norepinephrineCIt inhibits monoamine oxidase in the brainDIt blocks tyrosine hydroxylase in the periphery

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

The answer and the reasoning

A. It blocks peripheral conversion of levodopa to dopamine

Carbidopa inhibits aromatic L-amino acid decarboxylase outside the brain, so less levodopa is decarboxylated to dopamine in the periphery. More levodopa reaches the brain, allowing a lower dose and fewer peripheral side effects such as nausea. Because it cannot enter the brain, conversion to dopamine there continues.

Option by option

A. It blocks peripheral conversion of levodopa to dopamine (correct)
Correct. Inhibiting peripheral DOPA decarboxylase preserves levodopa for entry into the brain.
B. It converts dopamine to norepinephrine
The conversion of dopamine to norepinephrine is catalysed by dopamine beta-hydroxylase.
C. It inhibits monoamine oxidase in the brain
Inhibiting brain monoamine oxidase is the action of selegiline and is not what carbidopa does.
D. It blocks tyrosine hydroxylase in the periphery
Blocking tyrosine hydroxylase would reduce levodopa formation rather than protect supplied levodopa.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, It blocks peripheral conversion of levodopa to dopamine. Carbidopa inhibits aromatic L-amino acid decarboxylase outside the brain, so less levodopa is decarboxylated to dopamine in the periphery.

Why is "It converts dopamine to norepinephrine" not the answer?

The conversion of dopamine to norepinephrine is catalysed by dopamine beta-hydroxylase.

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

Inhibiting brain monoamine oxidase is the action of selegiline and is not what carbidopa does.

Why is "It blocks tyrosine hydroxylase in the periphery" not the answer?

Blocking tyrosine hydroxylase would reduce levodopa formation rather than protect supplied levodopa.

What concept does this question test?

It tests Amino acid metabolism: levodopa with carbidopa in Biochemistry. More levodopa reaches the brain, allowing a lower dose and fewer peripheral side effects such as nausea. Because it cannot enter the brain, conversion to dopamine there continues.

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 Amino acid metabolism: levodopa with carbidopa?

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.