A patient with a coagulation disorder is found to have a prolonged activated partial thromboplastin time (aPTT) and a normal prothrombin time (PT). Which of the following coagulation factors is most likely to be deficient?

A prolonged aPTT with a normal PT suggests a defect in the intrinsic pathway of coagulation, which involves factors VIII, IX, XI, and XII. Factor VIII deficiency is a common cause of this pattern and is associated with hemophilia A.