One of the main symptoms of Parkinson's disease is

A) impairment of initiating voluntary movement.
B) sudden fits of falling asleep in the middle of the day.
C) severe, sudden attacks of anxiety.
D) inability to understand spoken language.

One of the main symptoms of Parkinson's disease is actually impairment of initiating voluntary movement, also known as bradykinesia. This can include difficulty starting and stopping movements, as well as a general slowness of movement. Sudden fits of falling asleep, severe anxiety, and inability to understand language are not typically associated with Parkinson's disease.