A 72-year-old man with a 3-year history of progressive memory loss and difficulty recognizing family members is brought to the emergency department by his son. Over the past 6 hours, the patient has become acutely agitated, is unable to focus attention, and is seeing people in the room who are not present. His temperature is 38.5°C, heart rate 110/min, and blood pressure 145/90 mmHg. Urinalysis shows pyuria and positive nitrites. The son reports that the patient's baseline cognitive decline has been slow and steady, but this acute change occurred suddenly after he developed dysuria yesterday. What is the most likely diagnosis?