A 19-year-old man is brought to the hospital after injuring his knee playing football. The knee is swollen and painful to move. He has no prior history of bleeding disorders. Arthrocentesis of the knee reveals 20 cc of blood. Further investigations show that his platelets are 170,000/mL, PT is normal, PTT is elevated, bleeding time is normal, factor VIII is normal, factor IX is reduced, and the ristocetin cofactor assay is normal.For the above patient with a bleeding disorder, select the most likely diagnosis.