Which factor deficiency is characterized by both a prolonged PT and APTT?

The characteristic of a prolonged prothrombin time (PT) and activated partial thromboplastin time (APTT) indicates a deficiency in a factor that is involved in both the extrinsic and intrinsic pathways of the coagulation cascade. Factor X is a crucial component that sits at the convergence of these two pathways; it is activated by both the extrinsic pathway (with Factor VII) and the intrinsic pathway (with Factors IX and VIII).

When Factor X is deficient, both the PT and APTT will be prolonged due to the disruption of coagulation processes in both pathways. This dual prolongation is significant in identifying Factor X deficiency.

In contrast, deficiencies of Factors VIII, IX, and XI primarily affect only the intrinsic pathway, which typically results in a prolonged APTT but would not affect the PT. Therefore, these factors are not responsible for the observed prolongation in both tests that is seen in Factor X deficiency.