A 28 year-old male patient comes to your office and reports severe headache, nausea, vomiting, lethargy and weakness. You notice he has a slurred speech as he answers your questions. He reports he frequently has been having nose bleeds since his childhood. Unlike in other children, his nose bleeds never stopped. His gums often bleed when he brushes his teeth. He has family members with bleeding disorders. Occasionally, he takes aspirin for his headaches. Later in physical examination, you notice focal neurological deficits and minor bruises. CT head reveals an intracranial hemorrhage. Laboratory results show low platelet count, prolonged bleeding time, normal prothrombin time and normal activated partial thromboplastin time. Peripheral smear is notable for giant, enormously large megakaryocytes and platelets. Platelet aggregation studies demonstrate a reduced response to ristocetin that is not corrected by the addition of normal plasma. Of the following, which is the most likely diagnosis in this patient?
