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Imaging revealed diffuse pulmonary infiltrates; laboratory evaluation showed anti-myeloperoxidase p-ANCA positivity and negative anti-glomerular basement membrane and anti-nuclear antibody serologies. Renal biopsy demonstrated pauci-immune necrotising crescentic glomerulonephritis. She was diagnosed with MPA causing pulmonary-renal syndrome. Aggressive immunosuppressive therapy with intravenous methylprednisolone and rituximab induced remission, and renal function improved substantially. This case highlights the variable manifestations of MPA and the potential for misdiagnosis. Prompt ANCA testing and tissue biopsy are critical for diagnosis; early recognition and treatment of MPA can prevent end-organ damage.<\/jats:p>","DOI":"10.1136\/bcr-2026-272871","type":"journal-article","created":{"date-parts":[[2026,7,29]],"date-time":"2026-07-29T11:15:24Z","timestamp":1785323724000},"page":"e272871","update-policy":"https:\/\/doi.org\/10.1136\/crossmarkpolicy","source":"Crossref","is-referenced-by-count":0,"title":["The pneumonia that wasn\u2019t: evolving pulmonary-renal syndrome in microscopic polyangiitis"],"prefix":"10.1136","volume":"19","author":[{"ORCID":"https:\/\/orcid.org\/0009-0009-2380-5681","authenticated-orcid":false,"given":"Raksha Madhu","family":"Narasimhan","sequence":"first","affiliation":[{"name":"Department of Medical Education, University of Miami Leonard M Miller School of Medicine, Miami, Florida, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Aziz","family":"Akrama","sequence":"additional","affiliation":[{"name":"Department of Internal Medicine, University of Miami Leonard M Miller School of Medicine, Miami, Florida, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Mashli","family":"Fleurestil","sequence":"additional","affiliation":[{"name":"Department of Internal Medicine, University of Miami Leonard M Miller School of Medicine, Miami, Florida, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Maria","family":"Khan","sequence":"additional","affiliation":[{"name":"Department of Internal Medicine-Pediatrics, University of Miami Leonard M Miller School of Medicine, Miami, Florida, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Tiaosi","family":"Xing","sequence":"additional","affiliation":[{"name":"Department of Pathology & Laboratory Medicine, University of Miami Leonard M Miller School of Medicine, Miami, Florida, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Janelis","family":"Gonzalez","sequence":"additional","affiliation":[{"name":"Department of Internal Medicine, University of Miami Leonard M Miller School of Medicine, Miami, Florida, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"239","published-online":{"date-parts":[[2026,7,29]]},"reference":[{"key":"2026072904150990000_19.7.e272871.1","doi-asserted-by":"publisher","DOI":"10.2215\/CJN.02500317"},{"key":"2026072904150990000_19.7.e272871.2","doi-asserted-by":"publisher","DOI":"10.1053\/j.ajkd.2019.04.031"},{"key":"2026072904150990000_19.7.e272871.3","doi-asserted-by":"crossref","DOI":"10.1186\/1471-230X-13-42","article-title":"Microscopic polyangiitis complicated with ileal involvement detected by double-balloon endoscopy: a case report","volume":"13","author":"Fukushima","year":"2013","journal-title":"BMC Gastroenterol"},{"key":"2026072904150990000_19.7.e272871.4","doi-asserted-by":"crossref","DOI":"10.1016\/j.jtauto.2023.100191","article-title":"Autoantibodies to dsDNA in the diagnosis, classification and follow-up of patients with systemic lupus erythematosus","volume":"6","author":"Damoiseaux","year":"2023","journal-title":"J Transl Autoimmun"},{"key":"2026072904150990000_19.7.e272871.5","doi-asserted-by":"publisher","DOI":"10.1056\/NEJMoa0909905"},{"key":"2026072904150990000_19.7.e272871.6","doi-asserted-by":"crossref","unstructured":"Akiyama M , Takanashi S , Takeuchi T , et al . 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