Atypical Presentation of Microscopic Polyangiitis in an Elderly Woman.

Microscopic polyangiitis (MPA) is a rare, necrotizing vasculitis that primarily affects small-sized blood vessels, with a predilection for the upper and lower respiratory tract and the kidneys. It is commonly associated with the presence of perinuclear antineutrophil cytoplasmic antibodies (p-ANCA), often targeting myeloperoxidase (MPO), and typically presents with renal impairment, pulmonary symptoms, or systemic features such as fatigue and weight loss. We report the case of a 76-year-old Caucasian woman who presented with anemia, dark stools, and acute kidney injury (AKI) but lacked significant pulmonary symptoms. Her evaluation revealed chronic sinusitis on imaging and pauci-immune necrotizing glomerulonephritis (GN) on kidney biopsy, with perinuclear antineutrophil cytoplasmic antibody (p-ANCA) positivity and limited crescent formation. Despite the absence of overt respiratory or constitutional symptoms, her diagnosis of MPA was confirmed, and she was initiated on rituximab therapy with plans for outpatient rheumatology and nephrology follow-up. This case highlights the importance of considering vasculitis in elderly patients presenting with unexplained renal dysfunction and subtle upper respiratory findings, even in the absence of classic systemic features.