Stormy Course of Adult-Onset Still's Disease With Macrophage Activation Syndrome and Concurrent Membranoproliferative Glomerulonephritis: A Case Report.

Adult-onset Still's disease (AOSD) is a rare systemic inflammatory disorder of unknown etiology, characterized by fever, arthralgia, rash, and leukocytosis. Severe disease may be complicated by macrophage activation syndrome (MAS), a life-threatening hyperinflammatory state marked by cytopenias, hyperferritinemia, and multiorgan involvement. Renal manifestations in AOSD are uncommon and varied. We report the case of a 22-year-old Nigerian male who fulfilled the Yamaguchi criteria for AOSD and presented with features consistent with MAS, including cytopenias, hyperferritinemia, hypertriglyceridemia, and transaminitis. He also developed acute kidney injury with significant proteinuria. Renal biopsy demonstrated features consistent with membranoproliferative glomerulonephritis (MPGN). The patient was successfully treated with pulsed corticosteroids followed by oral steroids and tacrolimus, resulting in clinical and biochemical improvement. This case highlights the diagnostic complexity of AOSD complicated by MAS, particularly in the presence of atypical renal involvement. Early recognition and prompt initiation of immunosuppressive therapy are essential to improving outcomes.