<s>[INST] context: ref Robbins 7/e p127 , Harrison 17/e p346,9/e p120-121. Prothrombin time (PT) screens for the activity of the pro- teins in the extrinsic pathway (factors VII, X, II, V, and fibrinogen). The PT is performed by adding phospholip- ids and tissue factor to a patient's citrated plasma (sodium citrate chelates calcium and prevents spontane- ous clotting), followed by calcium, and the time to fibrin clot formation (usually 11 to 13 seconds) is recorded. Because factor VII is the vitamin K-dependent coagulation factor with the shoest half-life (roughly 7 hours), the PT is used to guide treatment of patients with vitamin K antagonists (e.g., coumadin). * Paial thromboplastin time (PTT) screens for the activity of the proteins in the intrinsic pathway (factors XII, XI, IX, VIII, X, V, II, and fibrinogen). The PTT is performed by adding a negatively charged activator of factor XII (e.g., ground glass) and phospholipids to a patient's citrated plasma, followed by calcium, and recording the time required for clot formation (usually 28 to 35 seconds). The PTT is sensitive to the anticoagulant effects of heparin and is therefore used to monitor its efficacy. Once thrombin is formed, it not only catalyzes the final steps in the coagulation cascade, but also exes a wide variety of effects on the local vasculature and inflammatory milieu; it even actively paicipates in limiting the extent of the hemostatic process [/INST] question: Which of the following is a procoagulation protein | options: A: Thrombomodulin B: Protein C C: Protein S D: Thrombin | answer: D </s>