A new pаtient аdmissiоn tо the step-dоwn unit is аdmitted for pneumonia, complicated by a history of heart failure. He lives independently; this is his first hospitalization in over a year. On the second evening, he becomes newly agitated. He is not oriented to place, day, or time and cannot explain why he is here "at the bank". He has fluctuating attention. Which of the following is the most likely diagnosis?