A 14-year-old patient complains of chest pain, temperature up to 38,5°C, breathlessness. He had acute tonsillitis 2 weeks ago. He is in grave condition. The skin is pale. Heart borders are dilated, heart sounds are quiet. Above total heart area you can hear pericardium friction sound. Electrocardiogram: the descent of QRS voltage, the inversion T. The liver is enlarged by 3 cm. ESR - 4 mm/h, ASL - 0 - 1260, C-reactive protein . Your diagnosis: