Adult-Onset Still's Disease as an Uncommon Cause of Fever of Unknown Origin.

Adult-onset Still's disease (AOSD) is a rare systemic autoinflammatory disorder characterized by fever, rash, arthritis, and hyperferritinemia, often presenting as a diagnostic challenge due to its nonspecific manifestations. We report the case of a 35-year-old previously healthy male who presented with a 20-day history of pruritic erythematous skin lesions, high-grade fever, severe odynophagia, and symmetric inflammatory polyarthritis. Laboratory evaluation revealed leukocytosis with neutrophilia, elevated acute-phase reactants, liver enzyme abnormalities, and hyperferritinemia (1,050 ng/mL). Infectious, autoimmune, and neoplastic causes were excluded. During hospitalization, the patient developed chest pain with electrocardiographic changes and elevated troponin levels, consistent with acute pericarditis. Skin biopsy demonstrated nonspecific inflammatory dermatitis. A diagnosis of AOSD was established based on clinical findings and Yamaguchi criteria. Initial treatment with systemic corticosteroids resulted in partial improvement; however, early relapse required escalation to tocilizumab, achieving complete clinical remission at two months. This case highlights the importance of considering AOSD in patients with fever of unknown origin and elevated inflammatory markers, as well as the role of early targeted therapy in improving outcomes.