A 72-year-old woman with hypertension and diabetes mellitus is admitted to the hospital with acute-onset confusion, disorientation, and agitation that developed over 6 hours. She has a fever of 38.5°C, tachycardia, and a urinary catheter in place for 3 days. Her mini-mental state examination score is 18/30. What is the most common cause of delirium in this hospitalized elderly patient?