Unmasking the Masquerade: A Case Report of Adult-Onset Still's Disease.

Adult-onset Still's disease (AOSD) is a rare, inflammatory condition commonly characterized by polyarthritis, rash, and fever. Its diagnosis is often missed entirely or delayed due to a lack of biomarkers and nonspecific symptoms. We report a case of a 40-year-old female presenting with an unresolving sore throat, small joint arthritis, worsening low-grade fever, and diffuse rash. After an extensive workup requiring multiple hospitalizations, she was ultimately diagnosed with AOSD. Significant symptom improvement was seen following the initiation of canakinumab and a brief course of steroids. Glucocorticoids and disease-modifying antirheumatic drugs are the mainstays of treatment. In those who present with fever of unknown origin and polyarthralgia, once other etiologies have been ruled out, steroids should be administered quickly for symptomatic control while AOSD workup is pursued, to avoid delayed diagnosis. As AOSD is a diagnosis of exclusion, extensive workup is mandatory and requires clinicians to persistently evaluate a plethora of objective data and redirect clinical reasoning to reach the correct diagnosis and avoid complications.