A 65-year-old man with diabetes mellitus type 2 and benign prostatic hyperplasia presents with fever (38.9°C), dysuria, and left flank pain. Urine culture grows E. coli resistant to fluoroquinolones but susceptible to cephalosporins and carbapenems. Blood cultures are pending. Serum creatinine is 1.8 mg/dL (baseline 1.2 mg/dL). What is the most appropriate next step in management?