A 67-yeаr-оld client presents tо the neurоlogy clinic with а 6-month history of progressive hаnd tremor at rest, stiffness, slowed movement, and a shuffling gait. On examination, the provider notes a pill-rolling tremor of the right hand, cogwheel rigidity, bradykinesia, and a masked facial expression. The provider suspects Parkinson's disease (PD). The nurse is reviewing the underlying pathophysiology and expected diagnostic findings with the client. Which diagnostic or pathophysiologic finding supports the diagnosis of Parkinson's disease?