You are answering a USMLE-style medical question. Question: A 60-year-old man with a history of osteoarthritis has been awaiting hip replacement surgery for 3 years. During his annual physical, he reports that he has been taking over the counter pain medications, but that no amount of analgesics can relieve his constant pain. Laboratory results reveal that his renal function has deteriorated when compared to his last office visit 2 years ago. Serum creatinine is 2.0 mg/dL, and urinalysis shows 1+ proteinuria. There are no abnormalities seen on microscopy of the urine. A renal biopsy shows eosinophilic infiltration and diffuse parenchymal inflammation. What is the most likely explanation for this patient's deterioration in renal function? Options: A) Focal segmental glomerulosclerosis B) Ischemic acute tubular necrosis C) Nephrotoxic acute tubular necrosis D) Toxic tubulointerstitial nephritis Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.