A 60-year-old man is brought to the clinic by his wife. She complains of tremors of his right hand and slowness of movements. The symptoms began 6 months ago and have been increasing. He has no history of serious illness nor head trauma. He takes no medications. Neurological examination shows a bilateral resting tremor more noticeable in his right hand, hypomimia, cogwheel rigidity, bradykinesia, and a festinating gait.

Which of the following is the most likely cause of his symptoms?