C3 Glomerulonephritis: A Rare Etiology of the Pulmonary Renal Syndrome.

C3 Glomerulopathy is a rare form of kidney disease due to dysregulation of the alternative complement pathway. We report a case of a college-aged woman with C3 glomerulonephritis (C3GN), presenting with the unexpected extrarenal manifestation of pulmonary hemorrhage. The patient presented with a nephritic urinary sediment and acute kidney injury after a recent infection. Kidney biopsy demonstrated focal endocapillary proliferative, crescentic, and necrotizing glomerulonephritis with bright glomerular C3 staining only. Electron microscopy revealed mesangial, intramembranous, and subendothelial deposits. After 2 doses of intravenous methylprednisolone, the patient developed spontaneous hemoptysis and respiratory compromise requiring emergent intubation. Bronchoscopy and computed tomography findings were consistent with diffuse alveolar hemorrhage. Notable laboratory results included C3, 40 (reference range, 75-175) mg/dL, and negative antinuclear antibody, antineutrophil cytoplasmic antibody, and anti-glomerular basement membrane serology results. As an outpatient, genetic testing revealed the presence of C3 glomerulopathy risk alleles. A diagnosis of C3GN complicated by pulmonary hemorrhage was made. There was initial response to treatment with steroids and mycophenolate mofetil; however, after repeated relapses of proteinuria and hematuria, treatment with eculizumab showed an initial response, but the patient subsequently became hemodialysis dependent. Our case highlights that C3GN can present with crescents and have other extrarenal manifestations such as pulmonary hemorrhage and should also be considered part of the differential diagnosis in patients presenting with pulmonary renal syndrome.