A 7-year-old boy is brought to the emergency department with a 10-day history of daily high fevers (up to 39°C/102.2°F), joint pain and a transient rash. His parents report that he appears fatigued and unwell during the fevers, which occur in the evening and resolve spontaneously, leaving him appearing well. They also noticed a pink rash on his trunk that comes and goes with the fever. Over the past few days, he has had increasing pain and swelling in his right knee and left elbow, with reduced movement. He is now holding his right knee and is unable to fully extend it. On examination, he has a salmon-colored macular rash, mild cervical lymphadenopathy, hepatosplenomegaly and warmth and swelling of the right knee and left elbow.

Laboratory values:

White blood cell count: 25 × 10⁹/L (normal: 4.5–11 × 10⁹/L)

Erythrocyte Sedimentation Rate: 105 mm/hr (normal <20 mm/hr)

C-reactive protein: 150 mg/L (normal <10 mg/L)