A 17-year-old presents with complaints of a recurrent nosebleed. He says that in the past his nosebleeds have continued for hours despite digital pressure. He says he was told he had a problem with bleeding. He occasionally has developed easy bruising and bleeding gums. Blood work reveals a CBC showing WBC 9, Hb 10, Platelets 95, PT 11, PTT 32, and INR 1.3. The bleeding time is prolonged, and his platelets appear not to aggregate. He denies the use of any medications. What is most likely his condition?