Which of the following is the PRIMARY mechanism by which tissue factor (TF) initiates the extrinsic pathway of coagulation in thrombosis?

Explanation

## Tissue Factor and Extrinsic Pathway Initiation **Key Point:** Tissue factor (TF, also called thromboplastin) is a membrane-bound cofactor that forms a complex with Factor VII (in the presence of Ca²⁺) to directly activate Factor X, bypassing the need for Factor XII contact activation. ### Mechanism of TF-Mediated Thrombosis Tissue factor initiates coagulation through the following sequence: 1. TF is exposed following endothelial injury or cell damage 2. TF binds to Factor VII (forming the TF-Factor VIIa complex) 3. This complex requires Ca²⁺ ions and phospholipid surfaces 4. The TF-Factor VIIa complex directly activates Factor X to Factor Xa 5. Factor Xa then converts prothrombin (Factor II) to thrombin (Factor IIa) **High-Yield:** The TF-Factor VII complex is the **fastest** route to thrombin generation and is responsible for initiating thrombosis in vivo. This is why TF is considered the primary initiator of the coagulation cascade in pathological thrombosis. ### Why TF is Critical in Pathological Thrombosis **Clinical Pearl:** Tissue factor is constitutively expressed in perivascular tissues (adventitia) and is exposed when there is endothelial damage. This explains why thrombosis occurs at sites of vascular injury rather than spontaneously in the bloodstream. **Mnemonic:** **TF-VII-X** — Tissue Factor binds Factor VII to activate Factor X (the extrinsic pathway's endpoint). [cite:Robbins 10e Ch 4]