A 70-year-old male with no prior medical history received gentamicin for infection treatment 10 days ago. Current findings: urine output 800ml, urinalysis (UA) shows Pro(+), granular casts present, blood urea nitrogen (BUN) 18.8mmol/L, creatinine (Cr) 373μmol/L, HGB 12.1g/L. The most likely cause of acute renal failure is: