A 75 years old man presents with problems walking that have developed over the past 2 months, consisting of slow gait, imbalance (especially on turning), short stride length, and gait initiation failure. He reports urinary frequency, occasional urge incontinence, headaches, and some memory loss. On examination, his symptoms are symmetric and much more prominent in the lower half of the body, with relative sparing of hand function, and normal facial expressiveness. Fundoscopic examination reveals bilateral papilledema. What is the most likely diagnosis?