A Diagnosis of Exclusion - Adult-Onset Still's Disease: A Case Report.

Adult-onset Still's disease (AOSD) is a rare systemic inflammatory disorder characterized by high spiking fevers, arthralgia or arthritis, and a distinctive salmon-colored evanescent rash. Diagnosis is clinical but can pose a challenge for providers given its rarity and variable clinical presentation. Here, the authors present the case of a 58-year-old Caucasian male exhibiting fever, myalgias, hyperferritinemia, polyarthralgia, and diffuse pruritic rash. While a rash is part of the typical presentation for AOSD, the diffuse pruritus seen in this patient is atypical. Several differentials were considered, and infectious, neoplastic, and alternative rheumatologic etiologies were excluded. Despite the pruritic nature of the patient's rash, the Yamaguchi criteria for AOSD was fulfilled. The patient achieved symptomatic improvement after treatment with NSAIDs, antihistamines, and corticosteroids. This case emphasizes the diagnostic challenge AOSD can pose, especially when the presenting symptoms mimic more common conditions such as viral infections, allergic reactions, or autoimmune dermatologic diseases. It reinforces the importance of considering AOSD in the differential diagnosis of patients with persistent fevers, rash, and arthralgia, particularly when infectious and autoimmune workups are unrevealing. Early recognition is crucial, as prompt initiation of therapy can significantly reduce morbidity and prevent complications.