Q. A 66-year-old man is brought to the physician by his wife because of being forgetful and because of seeing and hearing unreal things. She reports that the symptoms started two years ago and have rapidly worsened. Last week he was talking to the wall, and when he was asked about that he said he was talking to angels sent by God. He, unintentionally, kicks and punches while he is sleeping. On clinical examination, he has increased resistance against passive movement and tremors in both hands. MRI demonstrates cortical atrophy without evidence of any significant T2 white matter hyperintensities. What is the most likely diagnosis?