Review Question - QID 214325

A 45-year-old man is brought to his primary care physician by his wife due to tremors for about 1 month. His tremor affects only his right hand and worsens while watching television. His wife has noticed that his movements have been slow, which is especially apparent with ambulation for the past few weeks. He recently saw a headache specialist due to his worsening migraine associated with nausea and vomiting and received treatment. The patient is otherwise healthy. On physical examination, the patient has increased rigidity with passive upper extremity movement. His gait is notable for unsteadiness, mild dragging of his legs, and taking short steps while ambulating a few feet. Which of the following is most likely the cause of this patient's symptoms?