Q. A 65-year-old man is brought to the physician by his wife because of frequent falls and blurry vision over the past 4 months. He has smoked 1 pack of cigarettes daily for the past 40 years. On clinical examination, he has slurred speech with postural instability, his eye movement is limited when looking downward. He has increased resistance to passive arm flexion. On MRI there is a Hummingbird sign. What is the most likely diagnosis?