A 73-year-old man with a history of smoking, type 2 diabetes mellitus, and multiple strokes is brought to the physician by his daughter. He has had several falls during the past few months. There is no history of recent illness, lifestyle changes, or changes in medication. He has no orthostatic hypotension. Neurological examination shows only a shuffling gait, brisk tendon reflexes, and lower limb rigidity. Eye movements are normal. Which of the following is the most probable diagnosis?