A 61-Year-Old Man With Weakness and Gait Dysfunction

A man aged 61 years presented with chronic progressive weakness and gait dysfunction over a 4-year period. He had Ashkenazi Jewish ancestry and a history of memory loss, inappropriate laughter, dysarthria, and hypertension. Examination revealed impaired recall, extremity spasticity, distal sensory loss, and normal cerebellar function, and imaging showed hyperintensities in the periventricular white matter of the corticospinal tracts of the pons and medulla. What is your diagnosis?