Movement Disorders USMLE Step 1 Practice Question

A 68-year-old man presents to clinic with a 2-year history of progressive tremor that is most noticeable when his hands are at rest. He notes that the tremor improves when he reaches for objects or performs intentional movements. Over the past year, he has developed difficulty initiating walking and reports that his steps have become smaller and shuffling. His wife reports he has become slower at performing daily tasks but denies any memory problems or personality changes. On examination, he has a resting tremor in both hands, reduced arm swing bilaterally, cogwheel rigidity in the wrists, and a stooped posture. Brain MRI with and without contrast shows no focal lesions, atrophy, or signal abnormalities. Which of the following is the most likely diagnosis?