A 58-year-old man with chronic kidney disease (eGFR 28 mL/min/1.73 m²) is prescribed gentamicin 80 mg intravenously once daily for a urinary tract infection. After 5 days of therapy, his serum creatinine rises from 2.2 to 3.8 mg/dL, and urinalysis shows muddy brown casts. Gentamicin trough level is 8 µg/mL (normal <2 µg/mL). Which type of adverse drug reaction is this, and what is the primary mechanism?