ANCA-Negative Microscopic Polyangiitis With Gastrointestinal and Renal Involvement: A Diagnostic and Therapeutic Challenge

We present the case of a 64-year-old male with acute kidney injury (AKI) and gastrointestinal symptoms. Computed tomography (CT) revealed inflammatory changes in the lungs and colon, and push enteroscopy confirmed enteritis. Initially, AKI was attributed to GI losses causing prerenal azotemia, but persistent proteinuria prompted kidney biopsy, confirming anti-neutrophil cytoplasmic antibody (ANCA)-negative microscopic polyangiitis (MPA). While induction immunosuppression was planned, the patient developed necrotizing fasciitis requiring treatment deferral. Renal function ultimately normalized with steroids alone, highlighting the need for individualized therapy in seronegative vasculitis. This case underscores the diagnostic challenge of ANCA-negative MPA and the pivotal role of histopathology and multidisciplinary collaboration.