Which of the following is the PRIMARY mechanism by which tissue factor (TF) initiates the coagulation cascade in disseminated intravascular coagulation (DIC)?

Explanation

## Tissue Factor and DIC Pathogenesis **Key Point:** Tissue factor (TF) is the primary trigger of coagulation in DIC and initiates the extrinsic pathway by binding to Factor VII. ### Mechanism of TF-Mediated Coagulation Tissue factor (also called thromboplastin) is released from: - Damaged endothelium - Monocytes (activated by cytokines like TNF-α, IL-1) - Cancer cells (especially adenocarcinomas) TF binds to **Factor VII** (not Factor XII) to form the TF-Factor VIIa complex, which then: 1. Directly activates **Factor X** → Factor Xa 2. Factor Xa converts **Factor II (prothrombin)** → **Thrombin (Factor IIa)** 3. Thrombin amplifies coagulation by activating Factors V, VIII, XI, and platelets **High-Yield:** This is the **extrinsic pathway**, not the contact (intrinsic) pathway. The extrinsic pathway is the primary initiator of DIC in most clinical scenarios. ### Why This Matters in DIC Once TF-Factor VIIa is activated, the cascade becomes self-perpetuating because: - Thrombin activates Factor XI → Factor XIa (amplification loop) - Thrombin activates platelets → platelet aggregation and microthrombi formation - Widespread fibrin deposition consumes platelets and clotting factors → consumptive coagulopathy **Clinical Pearl:** In sepsis-induced DIC (the most common cause), endotoxin (LPS) triggers monocyte release of TF, explaining why gram-negative sepsis is a major DIC trigger.