A 58-year-old woman with a 25-year history of daily consumption of phenacetin-aspirin combination analgesics for chronic headaches presents with gross haematuria, renal colic, and progressive renal dysfunction (serum creatinine 3.4 mg/dL). Urine microscopy reveals sloughed necrotic tissue. CT imaging shows small scarred kidneys with shrunken papillae and calcified debris in dilated calyces. At autopsy, the bivalved kidney reveals the pathognomonic features of analgesic nephropathy. The structure marked **A** in the diagram—the yellow-tan necrotic papilla sloughing into the calyx—represents which of the following pathological processes?