Case History: Case I

A 67-year-old man reported difficulty walking for several years. Over the past year there have been several falls. He has a shuffling gait, stooped posture, and pill-rolling tremor in both hands at rest. In the image, one of the sections corresponds to this patient; the other is from a normal patient. What level of brainstem is shown and which section is abnormal? What is the abnormality?