An 11-year-old girl complains of pain in the joints, a fever of 38°C, and weakness. She fell ill 5 days ago. Objectively, she has a maculopapular rash on the bridge of her nose and on her face, the joints are not changed, the heart sounds are weakened. Blood pressure 100/80 mm Hg. Complete blood count: erythrocytes 26 1012/L, leukocytes - 3.7-109/L, platelets-12-109/L, ESR - 45 mm/hour, CRP (+++). General urinalysis: protein - 0.063 g/L, erythrocytes -15-20 in sight, leukocytes 10-14 in sight. What is the most likely diagnosis in this case?