A 7 y.o. boy suddenly felt pain in his right knee, it became edematic. The day before he took part in a crosscountry race. Family anamnesis has no data about hemophilia and bleeding sickness. Objectively: body temperature is 37,5°C. The knee is painful, hot to the touch, edematic with local tissue tension over it. Blood count: Hb- 123 g/L, leukocytes - 5,6 × 103 * * * * * 9/L, thrombocytes -354 × 109/l, prothrombin time -12 seconds (normally 10-15 seconds), partly activated thromboplastin time - 72 seconds (normally 35-45 seconds). Hemorrhage time is normal, VIII:C factor is 5% of norm. What is the most probable diagnosis?