You are answering a USMLE-style medical question. Question: Ten days after undergoing a cadaveric renal transplant, a 21-year-old man has pain in the right lower abdomen and generalized fatigue. During the past 2 days, he has had decreasing urinary output. He passed 5 liters of urine on the second postoperative day and serum studies showed a creatinine concentration of 2.1 mg/dL. He has adult polycystic kidney disease and was on maintenance hemodialysis for 2 months prior to transplantation. Current medications include azathioprine, cyclosporine, prednisone, and enalapril. He appears lethargic. His temperature is 37.8°C (100°F), pulse is 101/min, and blood pressure is 164/94 mm Hg. Examination shows a healing surgical incision in the right lower abdomen. The graft site shows tenderness to palpation. His blood urea nitrogen concentration is 52 mg/dL, creatinine concentration is 4.1 mg/dL. A biopsy of the transplanted kidney shows tubulitis. C4d staining is negative. Which of the following is the most likely responsible for this patient's findings? Options: A) Recipient T-cells in the graft B) Drug-induced nephrotoxicity C) Fibrosis of the glomerular vessels D) Newly formed anti-HLA antibodies " Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.