A 72-year-old male is referred for psychiatric evaluation of suspected depression. His wife reports he has become increasingly apathetic and withdrawn over the past 8 months, with reduced spontaneous speech and loss of interest in hobbies. He has been falling frequently due to what she describes as a shuffling, wide-based gait that developed gradually. She also notes he has had several episodes of urinary incontinence over the past 3 months, which he attributes to not being able to reach the bathroom in time. On mental status examination, he is slow to respond, demonstrates psychomotor retardation, and scores 24/30 on the MMSE with deficits primarily in recall and executive function tasks. CT head reveals enlarged lateral ventricles out of proportion to the degree of cortical atrophy, with an Evans index of 0.35. Which of the following is the most likely diagnosis?