A 72-year-old man with a 3-year history of progressive memory loss and difficulty with complex tasks is brought to the hospital by his son. Over the past 3 days, he has become acutely confused, agitated, and is unable to recognize his family members. He is disoriented to time and place. His vital signs are: BP 140/90 mmHg, HR 102/min, RR 22/min, Temperature 38.5°C. Urine dipstick shows leukocyte esterase and nitrites. Blood cultures are pending. Cognitive testing reveals marked inattention and fluctuating consciousness. What is the most likely acute diagnosis superimposed on his baseline condition?