A 4-year-old boy has a history of bleeding. His 12-year-old sister has no evidence of a bleeding disorder. His mother's brother died at the age of 3 from hemorrhage. Laboratory results: PFA Normal, APTT: 64.0, PT: 10.5, TT: 15 seconds, Mixing study 35 seconds. Which of the following tests should be done next Platelet aggregation, Circulating anticoagulant, Factor VIII, Factor XII? Why? please explain.