An 8-year-old girl complains about joint pain, temperature rise up to 38°C, dyspnea. Objectively: the left cardiac border is deviated by 2,5 cm to the left, tachycardia, systolic murmur on the apex and in the V point are present. Blood count: leukocytes - 20,0 × 109/l, ESR - 18 mm/h. What sign gives the most substantial proof for rheumatism diagnosis?