Adult-Onset Still's Disease Presenting With Persistent Hyperpigmented Scaling Rash in a Young Ethiopian Woman: A Case Report From a Resource-Limited Setting.

Background: Adult-onset Still's disease (AOSD) is a rare systemic autoinflammatory disorder characterized by high-spiking fever, arthritis, rash, and systemic inflammation. Because there is no single diagnostic test, diagnosis relies on compatible clinical features, fulfillment of classification criteria, and exclusion of infectious, malignant, and autoimmune conditions. Atypical cutaneous manifestations may further delay recognition, particularly in resource-limited settings. Case Presentation: We report the case of a 22-year-old Ethiopian woman who presented with a five-month history of inflammatory polyarthritis, daily spiking fever, sore throat, and a persistent hyperpigmented scaling rash rather than the classic evanescent salmon-colored eruption. Laboratory evaluation demonstrated marked neutrophilic leukocytosis, elevated inflammatory markers, abnormal liver enzymes, and hyperferritinemia (1,675 μg/L). Echocardiography revealed mitral valvulitis with mild pericardial effusion. Infectious, autoimmune, and rheumatologic conditions, including acute rheumatic fever, rheumatoid arthritis, systemic lupus erythematosus, viral hepatitis, HIV infection, and other competing diagnoses, were excluded through clinical assessment and laboratory investigations. The patient fulfilled the Yamaguchi classification criteria for AOSD, while markedly elevated serum ferritin provided supportive evidence for the diagnosis. She was treated with intravenous pulse methylprednisolone followed by oral prednisolone and methotrexate, resulting in rapid resolution of fever, marked improvement in joint symptoms, fading of the skin lesions, and complete resolution of the pericardial effusion at one-month follow-up. Conclusion: This case highlights the diagnostic challenges of AOSD when atypical dermatologic manifestations occur, particularly in resource-limited settings where infectious diseases are frequently prioritized in the differential diagnosis. Persistent hyperpigmented scaling lesions should not exclude consideration of AOSD when accompanied by characteristic systemic features. Although elevated serum ferritin is not diagnostic, it can provide valuable supportive evidence when interpreted alongside clinical findings and after exclusion of alternative diagnoses.