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
A 19-year-old man has had recurrent bleeding occur in his knee when playing contact sports. He has no history of spontaneous bleeding, but his brother had similar problems. Consultation with a specialist reveals that he has "mild" hemophilia A. Which of the following factor abnormalities is consistent with this diagnosis? 
Select and explain. 
A. abnormal factor VIII function, B. decreased levels of functional factor VIII, C. decreased factor IX level, D. decreased von Willebrand factor
B: decreased levels of functional factor VIII
Answer: B: decreased levels of functional factor VIII 
Explanation: In hemophilia A, there are decreased levels of functioning factor VIII. This can be due to reduced amounts of normal VIII, the presence of a functionally abnormal protein, or a combination of both. The activated partial thromboplastin time (APTT) is prolonged, and the prothrombin consumption test is abnormal. The prothrombin time, thrombin clotting time, and bleeding time are usually normal.