In the intrinsic pathway, factor VIII combines with which factor to form the complex that activates factor X?

1. Factor XII starts a series of reactions that activates factor XI. 2. Activated factor XI in turn activates factor IX. 3. Factor VIII, supplied by the platelets and endothelial cells, then combines with factor IX. 4. That enzyme complex activates factor X and hands the process to the common pathway. 5. Factor XI acts one step earlier, so pairing it with factor VIII skips a stage. 6. Factor VII belongs to the extrinsic pathway only. 7. This VIII with IX pairing also explains why hemophilia A and hemophilia B, deficiencies of those two factors, produce clinically similar bleeding. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 18 "The Cardiovascular System: Blood", section 18.5 Hemostasis_