Granulomatosis With Polyangiitis: A Case of a 62-Year-Old Female Presenting With a Fever of Unknown Origin.

Fever of unknown origin (FUO) continues to present diagnostic challenges despite advancements in medical science. Granulomatosis with polyangiitis (GPA), a rare antineutrophil cytoplasmic antibody (ANCA)-associated small-vessel vasculitis characterized by necrotizing granulomas, is an uncommon but important cause of FUO. This case report describes a 62-year-old female presenting with prolonged fever, malaise, and nonspecific systemic symptoms, ultimately diagnosed with GPA after extensive investigations, including positive C-ANCA and anti-proteinase-3 serology and imaging revealing a cavitating lung lesion. The patient's symptoms improved following immunosuppressive therapy with corticosteroids and rituximab. This case highlights the necessity of considering vasculitis, particularly GPA, early in the differential diagnosis of FUO to enable timely treatment and prevent serious complications.