An autopsy is performed at the request of the family after the death of an 89-year-old man. During sectioning of the brain, the substantia nigra is noted to have significant degeneration in the area of the pars compacta. This man most likely suffered from which of the following?
A) Foville's syndrome B) Tourette's syndrome C) Wilson's disease D) Parkinson's disease E) Huntington's disease