Monoclonal Gammopathy With Masked IgG Kappa Deposits in a Patient With Systemic Lupus Erythematosus: A Case Report.

We present the case of an 18-year-old female with a recent influenza B infection, anemia, fatigue, arthralgias, obesity, and a new diagnosis of systemic lupus erythematosus. Initial evaluation revealed proteinuria suggestive of renal involvement. Kidney biopsy confirmed membranous glomerulonephritis with masked IgG deposits (MGMID), despite negative paraprotein studies and lack of classic immunofluorescence staining. Treatment with mycophenolate mofetil led to significantly improved proteinuria without corticosteroids. This unique case underscores the importance of IF-P in patients with unclear renal histopathology and adds to the limited literature on MGMID occurring in the context of well-defined autoimmune disease.