A 69 years old man presents with a one year history of mild slowness and loss of dexterity. His handwriting has become smaller, and his wife feels his face is less expressive and his voice is softer. Over the last few months he has developed a subtle tremor in the right hand, noted while watching television. His symptoms developed insidiously but have mildly progressed. He has no other medical history, but he has noted some mild depression and constipation over the last 2 years. His examination demonstrates hypophonia, masked facies, decreased blink rate and rigidity. An intermittent right blink rate rigidity. An intermittent right upper extremity resting tremor is noted while he is walking. What is the most likely diagnosis?