Question

L-Dopa is combined with carbidopa in the treatment of parkinsonism to:

• A. Decrease the efficacy of levodopa
• B. Inhibit peripheral decarboxylation of levodopa
• C. Increase the dose of levodopa required
• D. Inhibit conversion of levodopa to dopamine in the CNS

Hint:

Carbidopa cannot cross the blood brain barrier, so where exactly does it block the enzyme?

Answer 1

The Correct Option is B

Step 1: Recall the problem with plain levodopa. Levodopa is converted to dopamine by the enzyme dopa decarboxylase. If this happens in the peripheral tissues, most of the drug turns into dopamine before reaching the brain, causing nausea, vomiting, and cardiac effects, and very little reaches the CNS.
Step 2: Recall carbidopa's action. Carbidopa inhibits dopa decarboxylase but cannot cross the blood brain barrier. So it blocks the enzyme only in the periphery, sparing the brain.
Step 3: See the benefit. By stopping peripheral conversion, more levodopa survives to enter the brain, where it is converted to dopamine and relieves Parkinson symptoms. This lets the levodopa dose be lowered and reduces peripheral side effects. So the purpose is to inhibit peripheral decarboxylation of levodopa.
Step 4: Rule out the rest. Carbidopa increases, not decreases, efficacy (A wrong). It reduces, not increases, the required dose (C wrong). It does not act in the CNS, so it does not block central conversion (D wrong).
Answer: Option B (Inhibit peripheral decarboxylation of levodopa).