Secondary Membranous Nephropathy and Multifactorial AKI in a Patient with Breast Cancer

Introduction: Acute kidney injury (AKI) is a common, serious complication in cancer patients. This case highlights the diagnostic challenges in evaluating AKI in an oncology patient, especially when it may result from multiple factors including medications, hemodynamic instability, infection, or paraneoplastic processes. Membranous nephropathy has been associated with cancer progression and immune checkpoint inhibitors. Case Description: A 53-year-old female with triple-negative stage IV breast cancer and history of left upper extremity DVT presented with fever and dyspnea. CT chest revealed a partially occlusive pulmonary embolism. She was septic, hypotensive, and treated with IV vancomycin and piperacillin/tazobactam. Her baseline creatinine of 0.8 mg/dL rose to 2.0 mg/dL with subnephrotic proteinuria. She was receiving Carboplatin, Taxol, and Pembrolizumab. Nephrology was consulted for AKI. Biopsy showed acute tubular necrosis and secondary membranous nephropathy. PLA2r, THSD7A, and NELL1 antibodies were negative. The patient became anuric, required dialysis, and unfortunately did not recover renal function. She later expired. Discussion: This case illustrates multifactorial AKI in a patient with advanced breast cancer. Despite supportive care and discontinuation of nephrotoxins, renal function declined, emphasizing the value of biopsy in unclear AKI cases. The presence of secondary membranous nephropathy raised suspicion for a paraneoplastic process. Retrospective review showed prior proteinuria, suggesting that earlier nephrologic workup may have uncovered renal involvement sooner and possibly earlier cancer detection. Timely nephrology involvement and a broad differential are crucial in oncology patients with AKI.