Acute Adult-Onset Still Disease Presenting as Fever of Unknown Origin and Severe Myalgia in a Resource-Limited Tertiary Hospital in Brazil.

This case highlights the diagnostic challenges of managing a patient who initially presented with a fever of unknown source and subsequently met the criteria for fever of unknown origin (FUO) after more than 3 weeks of persistent fever despite an extensive diagnostic workup in a resource-limited tertiary hospital within Brazil's public health system. A man in his sixth decade presented with prolonged high-grade fever and severe lower-limb myalgia, initially raising suspicion for leptospirosis due to rural exposure and occupational history. Extensive evaluation-including blood cultures, serologies, and imaging screening-revealed no infectious or malignant cause. Persistent neutrophilic leukocytosis, marked hyperferritinemia, and elevated inflammatory markers suggested a systemic autoinflammatory disorder. A trial of low-dose prednisone produced partial improvement, and rheumatology consultation supported the diagnosis of adult-onset Still disease (AOSD) based on Yamaguchi criteria. Corticosteroid escalation resulted in the rapid resolution of fever and myalgia. This case underscores the importance of structured reasoning, systematic exclusion of common causes, and careful attention to laboratory patterns for timely diagnosis in resource-limited settings.