A 78-year-old man presents to the neurology clinic with complaints of slowness. He is accompanied by his wife who reports that he has been progressively slowing down in his movements and facial expressions. The symptoms have worsened rapidly since he experienced dizziness a few weeks ago. The patient has a medical history of type two diabetes, hypertension, high cholesterol, and a previous hemicolectomy for diverticulitis. He is currently taking ramipril, atorvastatin, paracetamol, amlodipine, metformin, prochlorperazine, and gliclazide. On examination, he exhibits a coarse bilateral tremor at rest, rigidity in both arms, and a slow gait. What is the most likely diagnosis?