Adult-Onset Still's Disease Presenting as Fever of Unknown Origin: A Case Report.

Adult-onset Still's disease (AOSD) is a rare systemic autoinflammatory disorder that should be considered in patients presenting with fever of unknown origin (FUO). Diagnosis is challenging due to overlapping features with infectious, autoimmune, and hematological conditions. We report the case of a 19-year-old female presenting with a six-day history of persistent fever, later developing an evanescent rash and migratory polyarthralgia. Laboratory findings included cytopenias, elevated inflammatory markers, and markedly elevated ferritin levels. After ruling out infectious, autoimmune, and neoplastic etiologies, AOSD was diagnosed based on the Yamaguchi classification criteria. Initial treatment with corticosteroids led to partial clinical improvement, but persistent articular symptoms required escalation to anakinra, resulting in complete remission. This case reinforces that AOSD should be considered in the differential diagnosis of FUO, particularly in the presence of quotidian fever, rash, polyarthralgia, and marked hyperferritinemia. Early recognition and appropriate treatment, including biological therapy, are essential to control disease activity and prevent complications.