Salmon-Pink Macular Rash — Still Disease

A 7-year-old girl presents with a 3-week history of daily high-grade fever (39.5–40.5°C), polyarthritis affecting knees and ankles, and a rash that appears on her trunk and proximal arms during fever spikes but completely disappears when her temperature normalizes. The feature marked **C** in the diagram—the evanescent nature of the rash—is a hallmark of which systemic inflammatory condition?