An 18-year-old man develops excessive bleeding 2 hours after wisdom tooth extraction. He has a history of easy bruising after playing sports, and of minor cuts that rebleed. His examination is normal, except for the tooth extraction site, which is still oozing blood. His hemoglobin is 14.8 g/dL, platelets 230,000 mL, PT is normal, and partial thromboplastin time (PTT) is elevated. A bleeding time is normal, factor VIII level is reduced, factor IX is normal, and ristocetin cofactor assay is normal.For the above patient with a bleeding disorder, select the most likely diagnosis.