Adult-Onset Still's Disease Presenting With Leukopenia and Pulmonary Infiltrates: A Mimicker of Infection and Malignancy.

Adult-onset Still's disease (AOSD) is a rare systemic inflammatory disorder that often mimics infections, tickborne diseases, or malignancies, leading to diagnostic delays. We present a 26-year-old male with persistent fever, arthralgia, lymphadenopathy, and elevated inflammatory markers unresponsive to antibiotics. Extensive infectious and autoimmune workup was largely negative except for positive antinuclear antibody (ANA) and ribonucleoprotein (RNP). Development of a maculopapular rash with spiking fevers and elevated ferritin (1240 ng/mL) met Yamaguchi's criteria for AOSD. Initiation of corticosteroids resulted in the rapid resolution of fever. This case highlights that AOSD should remain a diagnostic consideration in fever of unknown origin, even with positive autoantibodies.