Neurologic examination

A 69-year-old woman is brought to the physician by her son because of a 2-month history of memory problems, difficulty walking, and urinary incontinence. He says,” She always been very sharp, but recently she’s had a lot of trouble functioning.” She has forgotten on several occasions to turn off the stove when cooking, and she once forgot she had turned on the bathtub until she noticed water coming down the walls from the upstairs bathroom. She also is unsteady on her feet and has fallen at home on several occasions. She reports that her mood is fine. Physical examination shows mild nystagmus. She has a broad-based gait and cannot walk in a straight line without assistance. Neurologic examination shows no other focal findings. The mini-Mental State Examination score is 20/30. Which of the following is the most likely diagnosis?