A 68-year-old man with diabetes mellitus type 2 presents with fever, dysuria, and flank pain for 3 days. Urine culture grows Gram-negative rods. Serum creatinine is 1.2 mg/dL (baseline 0.9 mg/dL), and eGFR is 58 mL/min/1.73m². The patient is started on gentamicin 5 mg/kg IV once daily. On day 5 of therapy, serum creatinine rises to 1.8 mg/dL, urine output drops to 400 mL/day, and the patient develops tinnitus. Which of the following best explains the adverse effect observed and represents the most appropriate next step?