Membranous nephropathy secondary to very early-onset inflammatory bowel disease in a 3-year-old boy: case report.

Membranous nephropathy (MN) is uncommon in children overall, but secondary MN is relatively common in younger children. Inflammatory bowel disease (IBD) can be complicated by kidney disease, but IBD complicated by MN is rarely reported. A 3-year-old boy diagnosed with very early onset IBD (VEO-IBD) a year earlier was incidentally found to have proteinuria via urine screening system. Laboratory tests revealed nephrotic syndrome with microscopic hematuria and signs of ongoing inflammation associated with IBD activity. Kidney biopsy showed mesangial cell proliferation and dense subepithelial immune complex deposits, along with negative PLA2R staining, which supported a diagnosis of secondary MN. Oral steroid therapy was initiated for its potential benefit for both nephrotic syndrome and the underlying VEO-IBD. As a result, four months later, proteinuria and hematuria had resolved, and kidney function remained stable. This is the youngest reported patient with MN associated with IBD. Routine urinalysis is suggested by this case to be useful for detecting kidney complications of IBD, including MN. Moreover, steroid therapy is likely to be useful for MN secondary to IBD. Our report suggests that potential kidney complications should also be considered in patients with VEO-IBD.