A 76-year-old woman presents with acute confusion, agitation, and visual hallucinations starting over the last 2 days. She has a background of mild cognitive impairment and Parkinson's disease treated with levodopa/carbidopa. She recently started oxybutynin for urinary incontinence. Her vital signs are stable apart from a temperature of 37.9°C. Urine dipstick is positive for leucocytes and nitrites. What is the most likely cause of her delirium?