A 7-year-old boy suddenly developed pain and edema in the right knee. The day before, at school, he took part in cross-country skiing. There is no family history of hemophilia or susceptibility to bleeding. His body temperature is 37.5°C. The knee is painful on palpation, hot to the touch, and has edema with local tissue tension above it. In the blood: Hb – 123 g/L, leukocytes – 5.6 · 109/L, platelets – 354 · 109/L, prothrombin time – 12 seconds (normal range is 10–15 seconds), activated partial thromboplastin time – 72 seconds (normal range is 35–45 seconds). Bleeding time is normal, the factor VIII levels constitute 5% of normal value. Make the right diagnosis: