Question

A patient on carbidopa plus levodopa for 10 years now has a wearing-off effect. Which drug should be added to restore the action?

• A. Tolcapone
• B. Amantadine
• C. Rasagiline
• D. Benzhexol

Hint:

Block the second peripheral enzyme that destroys levodopa to make each dose last longer.

Answer 1

The Correct Option is A

Step 1: After a decade of levodopa-carbidopa, the response becomes pulsatile and short, the dose stops covering the full inter-dose interval. The therapeutic goal is to make each levodopa dose last longer rather than to add a fresh dopaminergic mechanism.

Step 2: Levodopa is degraded peripherally by two enzymes: dopa decarboxylase (already blocked by carbidopa) and COMT. Adding a COMT inhibitor closes the second escape route. Tolcapone does this, and because it has a longer half-life and crosses the blood-brain barrier, it improves levodopa bioavailability and reduces its clearance, translating into longer 'on' periods and less 'off' time.

Step 3: The alternatives miss the target: rasagiline blocks MAO-B, amantadine is reserved for dyskinesias, and benzhexol is an antimuscarinic chiefly for tremor. The drug that directly restores and prolongs the failing levodopa effect is the COMT inhibitor.

\[\boxed{\text{Tolcapone}}\]