Which of the following clotting factors is the first factor common to both intrinsic and extrinsic pathways?a. Factor I (fibrinogen)
The intrinsic pathway begins with the activation of factor XII that subsequently activates factors XI, IX, and VII. In this pathway, each of the primary factors is "intrinsic" to the circulating plasma, whereby no surface is required to initiate the process. In the extrinsic pathway, tissue factor (TF) is released or exposed on the surface of the endothelium, binding to circulating factor VII, facilitating its activation to VIla. Each of these pathways continues on to a common sequence that begins with the activation of factor X to Xa (in the presence of VIlla). Subsequently, Xa (with the help of factor Va) converts factor II (prothrombin) to thrombin and then factor I (fibrinogen) to fibrin. Clot formation occurs after fibrin monomers are cross-linked to polymers with the assistance of factor XIII.