A nephritic puzzle: C3-dominant glomerulonephritis as a sentinel of hidden autoinflammatory disease.

Membranoproliferative glomerulonephritis (MPGN) is among the most challenging glomerulonephritides to diagnose and manage, given its clinico-immunopathologic heterogeneity. Although recent advances have introduced new therapies for complement-mediated and immune-complex MPGN, a substantial subset of cases remains poorly understood and lacks effective treatment. We report a pediatric case of biopsy-confirmed C3-dominant MPGN without evidence of complement dysregulation, with disease flares triggered by febrile episodes. Genetic testing identified underlying hyper-IgD syndrome (HIDS), a rare autoinflammatory disorder characterized by recurrent fevers and immune dysregulation. The patient's MPGN was refractory to conventional immunosuppressive therapy but achieved remission with IL-1β blockade that targets HIDS. This case highlights the importance of considering alternative etiologies-including autoinflammatory diseases-in patients with MPGN when complement dysregulation is not evident.