A Woman with Fatigue and Leg Pain

Examination was notable for low-grade fever, quadriceps weakness, and patchy, non–length-dependent sensory loss of her legs. Serial labs showed marked inflammation, mixed liver injury, and kidney injury with hematuria and proteinuria. After a negative workup for infection and malignancy, focus shifted toward an autoimmune process; pauci-immune necrotizing arteritis on kidney biopsy (Figure 1) and an elevated antineutrophil cytoplasmic antibody (ANCA) titer supported a diagnosis of microscopic polyangiitis.