A 62-year-old man from Delhi presents with a 3-year history of progressive tremor at rest in his right hand, rigidity, and bradykinesia. His wife reports he has become emotionally withdrawn and shows reduced facial expression. Neuroimaging is normal. On examination, he has a classic 'pill-rolling' tremor and cogwheel rigidity. Based on the presumed pathophysiology of basal ganglia dysfunction, what is the most appropriate immediate pharmacological intervention?