Hemophilia

Hemophilia A and hemophilia B are inherited coagulation disorders caused by deficiency of factor VIII and factor IX, respectively. Both factors participate in the intrinsic tenase complex: activated factor IX is the enzyme, and activated factor VIII is its cofactor. Together, they activate factor X efficiently enough to generate thrombin and stabilize a fibrin clot. When either factor is deficient, thrombin generation is inadequate. The initial platelet plug may form, but the deeper fibrin reinforcement is delayed or weak, so bleeding can continue or restart after the initial injury appears controlled. This mechanism explains the clinical pattern. Hemophilia causes bleeding into joints, muscles, and internal tissues. Petechiae, diffuse gingival oozing, and other primarily mucocutaneous findings point more toward a platelet disorder or von Willebrand disease than isolated hemophilia. Platelets are generally present and functional; the problem is the secondary coagulation phase that strengthens the plug. The condition is usually X-linked recessive, so it affects males more often because they have one X chromosome. A woman with a disease-causing variant may be a carrier but can also have clinically significant low...