Unmasking Granulomatosis With Polyangiitis Through a Large Pulmonary Cavity: A Case Report and Review of the Literature.

Granulomatosis with polyangiitis (GPA) is an antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis characterized by necrotizing inflammation of small- and medium-sized vessels. Although pulmonary involvement is common, isolated pulmonary presentations without upper airway or renal manifestations are uncommon and may mimic infection or malignancy, resulting in diagnostic delays. A 33-year-old male presented with four days of hemoptysis accompanied by fever, chills, unintentional weight loss, and bilateral ankle and knee arthralgias. Chest imaging demonstrated a large 7 × 8 × 8 cm thick-walled cavitary lesion in the right lower lobe. Given the radiographic appearance, an extensive infectious workup was pursued and proved negative, including evaluation for bacterial, fungal, viral, and mycobacterial pathogens. Autoimmune serologies revealed positive cytoplasmic antineutrophil cytoplasmic antibody (C-ANCA) and elevated proteinase-3 (PR3) antibodies. Bronchoscopy with transbronchial biopsy demonstrated nonspecific inflammatory changes without evidence of necrotizing granulomatous inflammation. Despite nondiagnostic histopathology, the combination of clinical presentation, radiographic findings, and highly specific serologic markers supported the diagnosis of GPA. The patient was treated with high-dose corticosteroids and rituximab, resulting in significant clinical improvement. This case highlights the diagnostic challenges of isolated pulmonary GPA presenting as cavitary lung disease. It emphasizes the importance of maintaining a broad differential diagnosis when evaluating cavitary pulmonary lesions and demonstrates the diagnostic value of ANCA serologies when tissue sampling is inconclusive. Early recognition and initiation of immunosuppressive therapy are critical to preventing disease progression and irreversible organ damage.