This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Blood, Subject name: Medicine
[{'content': '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. \nSelect and explain. \nA. von Willebrand disease, B. hemophilia A, C. hemophilia B, D. thrombotic thrombocytopenic purpura (TTP)', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Haemophilia A results from a deficiency or dysfunction of the factor VIII molecule. It is a sex-linked disease affecting 1 in 10,000 males. The PTT is prolonged, but the bleeding time is characteristically normal.', 'role': 'assistant'}]