Membranous-like glomerulopathy with masked IgG-k deposits in a pediatric patient with juvenile idiopathic arthritis.

Membranous-like glomerulopathy with masked IgG-kappa deposits (MGMID) is a rare entity described primarily among young females with previously diagnosed autoimmune diseases. We present a 12-year-old female with systemic juvenile idiopathic arthritis (sJIA) with persistent non-nephrotic range proteinuria despite normal kidney function. She underwent two kidney biopsies with the second ultimately confirming her diagnosis. The initial biopsy was suggestive of mild C3 glomerulonephritis (C3GN). She was started on an angiotensin-converting enzyme inhibitor (ACE-I) without improvement. Proteinuria progressed to the nephrotic range, prompting initiation of high-dose steroids followed by a steroid taper. Mycophenolate was added during steroid weaning due to ongoing proteinuria. Despite full-dose mycophenolate and ACE-I therapy, a repeat biopsy was performed due to lack of response and revealed MGMID. She remains on full-dose mycophenolate and lisinopril with significant improvement in her proteinuria.