Lessons of the month: ANCA-associated vasculitis - granulomatosis with polyangiitis: 'the great mimic'.

We describe the case of a 61-year-old woman who presented with progressive respiratory symptoms and imaging demonstrating multiple opacities in the right lung with hilar and mediastinal lymphadenopathy suggestive of multifocal adenocarcinoma. Subsequent biopsies were consistent with focal changes of organising pneumonia (OP) and no evidence of malignancy. She was treated with steroids for cryptogenic OP with limited response. There was clinical and radiological progression with new lung nodules, mediastinal and thoracic spinal canal infiltration. There was ongoing concern that clinical findings represented disseminated malignancy. Following further investigation and multidisciplinary respiratory and rheumatology review, a diagnosis of anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) - granulomatosis with polyangiitis (GPA) was confirmed. The case highlighted the multisystem nature of GPA with unusual dural and large vessel aortic and pulmonary trunk involvement. © Royal College of Physicians 2021. All rights reserved.