A 7-year-old boy suddenly developed pain and edema in his right knee. The day before, at school, he took part in crosscountry skiing. There is no family history of hemophilia or susceptibility to bleeding. Objectively, 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. Incomplete blood count: 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 35^15 seconds). Bleeding time is normal, the factor VIII levels constitute 5% of normal value. Make the diagnosis: