MSRA-1562

A 62-year-old man is being evaluated for cognitive impairment with concerns about a movement disorder. As part of his evaluation, he undergoes cross-sectional brain imaging and a dopamine Transporter Scan (DaTscan). The results show reduced dopaminergic activity in the substantia nigra bilaterally on the DaTscan and age-related changes on the MRI head. What is the most probable diagnosis?