All of the following statements regarding the intrinsic coagulation pathway are correct EXCEPT:

Explanation

## Intrinsic vs. Extrinsic Pathway: In Vivo Roles **Key Point:** The **extrinsic pathway** (initiated by Tissue Factor + Factor VII) is the primary pathway responsible for hemostasis in vivo, NOT the intrinsic pathway. The intrinsic pathway is a laboratory artifact of in vitro testing. ### Pathway Initiation and Physiological Importance | Pathway | Initiator | In Vivo Role | In Vitro Role | |---------|-----------|-------------|---------------| | **Extrinsic** | Tissue Factor (TF) + Factor VII | PRIMARY — triggered by tissue injury | Rapid activation in aPTT/PT | | **Intrinsic** | Contact activation (Factor XII) | MINIMAL — mainly amplification | Detectable in aPTT; prolonged aPTT | | **Common** | Factor X → Factor II → Fibrin | Convergence point | Both pathways converge | **High-Yield:** In vivo, Factor VII + Tissue Factor directly activate Factor X (extrinsic pathway), bypassing the entire intrinsic cascade. The intrinsic pathway's role is **amplification** of thrombin generation, not initiation of clotting. ### Intrinsic Pathway Components (All Correct) 1. **Factor XII (Hageman factor)** — initiates via contact with negatively charged surfaces (glass, phospholipids) 2. **Factor VIII** — acts as a cofactor for Factor IXa in the **tenase complex** (Factor IXa + VIIIa + PL + Ca²⁺) → activates Factor X 3. **Factor XI deficiency** — causes mild bleeding despite markedly prolonged aPTT (because extrinsic pathway compensates) **Clinical Pearl:** Patients with Factor XII deficiency have **no bleeding tendency** despite severely prolonged aPTT — this is the classic "discordance" that proves the intrinsic pathway is not essential for hemostasis in vivo. ### Why Option 3 Is Wrong The statement claims the intrinsic pathway is "primarily responsible for hemostasis in vivo" — this is **false**. The extrinsic pathway (TF + FVII) is the primary initiator. The intrinsic pathway amplifies but does not initiate clotting in vivo. [cite:Robbins 10e Ch 4]