A 55-year-old man with Parkinson's disease on levodopa-carbidopa develops sudden onset uncontrollable movements of his limbs 2 hours after his morning dose. This side effect is termed:

• A Wearing-off phenomenon due to declining dopamine levels before the next dose
• B Peak-dose dyskinesia caused by excessive striatal dopaminergic stimulation at drug peak ✓
• C Diphasic dyskinesia occurring at both rising and falling phases of plasma levodopa
• D Off-period dystonia from abrupt dopamine depletion causing sustained muscle contractions

Explanation

Peak-dose dyskinesia occurs at maximal plasma levodopa concentration (typically 1–2 hours after oral dose) due to excessive and pulsatile dopaminergic stimulation of supersensitive striatal D1/D2 receptors. It is the most common type of dyskinesia in Parkinson's disease and manifests as choreiform or ballistic involuntary movements. Management includes reducing individual levodopa doses, using extended-release formulations, or adding amantadine (which reduces NMDA-mediated dyskinesia).

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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