A 78-year-old resident of a care home develops acute confusion, agitation, and urinary incontinence over the past 24 hours. She has a history of Alzheimer's dementia, usually stable. Her observations are: Temp 37.9°C, HR 98 bpm, BP 130/70 mmHg, RR 20/min. A urine dipstick is positive for leukocytes and nitrites. What is the most likely cause of her acute deterioration?