A previously healthy 67-year-old man, who is experiencing an acute change in mental status, is brought to the emergency department by his family. There is no evidence in the initial history, physical examination, and laboratory studies to indicate substance intoxication or withdrawal, or to suggest another medical problem as the cause of his altered mental state. Over the course of 1 hour of observation, his level of alertness varies from alert but distractible, with apparent
auditory and visual hallucinations, to somnolent; he has difficulty sustaining attention to an examiner, and he cannot perform simple tasks such as serial subtractions or spelling words backwards. What is the most appropriate diagnosis?
Select one of the following: