A boy with hemophilia bleeds into his knee after a minor fall. Hemophilia A (factor VIII deficiency, X-linked) bleeds into joints and muscles — hemarthrosis swells the knee; repeated bleeds destroy the joint. Factor VIII concentrate (now recombinant) stops it; prophylactic infusions prevent. Contrast ITP — mucosal bleeding (gums, petechiae) from low platelets, not factors. The bleeding site diagnoses the system: joints/muscles = factors, skin/mucosa = platelets/vessels.