Hemophilia A accounts for about 80 percent of cases. Which clotting factor is deficient?

1. Hemophilia is a group of related disorders, each from inadequate production of a clotting factor. 2. Hemophilia A is the most common, at approximately 80 percent of cases. 3. It results from the inability to synthesise sufficient factor VIII. 4. Factor IX deficiency gives hemophilia B, the second most common form at about 20 percent. 5. Factor XI deficiency gives hemophilia C, a rare autosomal condition. 6. The letters therefore run A, B, C against factors VIII, IX, XI, and the missing X in that list is the reason the mapping has to be learned rather than guessed. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 18 "The Cardiovascular System: Blood", section 18.5 Hemostasis_