A 62-year-old man from Delhi presents with a 3-year history of progressive slowness of movement and tremor in his right hand at rest. On examination, he has a masked facies, cogwheel rigidity in both upper limbs, and a shuffling gait with reduced arm swing. His Mini-Cog score is 4/5. He is currently on no medications. Which of the following is the most appropriate first-line pharmacological agent to initiate in this patient?