Microscopic polyangiitis (MPA) is a rare antineutrophil cytoplasmic antibodies (ANCA)-associated vasculitis that poses unique diagnostic and therapeutic challenges in seniors. We describe a 90-year-old female with diabetes, hypertension and heart failure who presented with anasarca and rapid renal failure. Evaluation revealed myeloperoxidase-ANCA positivity, and renal biopsy confirmed pauci-immune crescentic glomerulonephritis. Her course was complicated by anemia and deep vein thrombosis. She was managed with high-dose corticosteroids, rituximab, and supportive hemodialysis, achieving improvement in renal function. This case highlights how comorbidities may obscure recognition of vasculitis and how assessment of frailty and baseline function are essential in weighing risks of immunosuppression in older adults. Clinicians should maintain vigilance for ANCA-associated vasculitis in older adults with unexplained renal decline and tailor therapy to balance disease control with vulnerability to treatment toxicity.