Which mechanism explains the benefit of entacapone when added to levodopa-carbidopa in a patient with Parkinson disease who has wearing-off phenomena?

• A It blocks peripheral catechol-O-methyltransferase, prolonging levodopa half-life ✓
• B It inhibits monoamine oxidase B in the striatum
• C It is a dopamine receptor agonist acting directly on D2 receptors
• D It promotes presynaptic dopamine release from surviving nigral neurons

Explanation

Entacapone is a peripheral, reversible COMT inhibitor. By blocking conversion of levodopa to 3-O-methyldopa outside the brain it increases the fraction of each levodopa dose reaching the striatum and smooths plasma levels, reducing wearing-off. Selegiline and rasagiline are MAO-A inhibitors, pramipexole is a direct agonist, and amantadine relates to release mechanisms. Entacapone itself has no central dopaminergic action.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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