A 42-year-old woman gives birth to a neonate with multiple congenital abnormalities. Physical findings included a flat facial profile, slanted eyes, epicanthal folds, Brushfield spots, short nose, short neck, dysplastic ears, clinodactyly, a large protruding tongue, and a pronounced heart murmur. As an adult, the brain of the patient will show histopathologic changes that are seen in patients with which of the following neurologic diseases?