Fever of unknown origin: adult-onset Still’s disease as the hidden culprit—a case report

Adult-onset Still's disease (AOSD) is a rare systemic autoinflammatory disorder characterized by prolonged fever, arthritis, and a maculopapular rash. Due to its nonspecific presentation, it often mimics infections, malignancies, and autoimmune diseases, making diagnosis challenging. We report the case of a 19-year-old Caucasian, Nepalese female presenting with persistent fever, joint pain, rash, and systemic symptoms. Laboratory investigations revealed leukocytosis, anemia, and elevated liver enzymes, while imaging showed lymphadenopathy and mild hepatomegaly. Extensive evaluation ruled out infectious, autoimmune, and malignant causes. Significantly elevated serum ferritin and C-reactive protein supported the diagnosis of AOSD based on Yamaguchi criteria. The patient responded dramatically to corticosteroid therapy, with full resolution of symptoms and normalization of laboratory findings within 2 weeks. This case highlights the importance of considering AOSD in patients with prolonged fever and systemic features after common causes are excluded. Timely diagnosis using clinical criteria and inflammatory markers can facilitate prompt treatment and favorable outcomes.