A 52-year-old man with a history of recurrent deep vein thrombosis is being evaluated for a hypercoagulable state. His coagulation studies show: aPTT 28 seconds (normal 25–35), PT 12 seconds (normal 11–13.5), platelet count 250,000/μL, and fibrinogen 350 mg/dL. All of the following are true regarding the common pathway of coagulation EXCEPT:

Explanation

## Common Pathway of Coagulation **Key Point:** The common pathway is the final convergence point where both the extrinsic and intrinsic pathways merge. It is NOT initiated by the extrinsic pathway alone — both pathways feed into it simultaneously. ### Convergence of Pathways ```mermaid flowchart TD A[Extrinsic Pathway<br/>TF + Factor VII]:::action --> B[Factor X Activation]:::action C[Intrinsic Pathway<br/>Factor XIa + Factor IXa + Factor VIII]:::action --> B B --> D[Factor Xa + Factor V<br/>Prothrombinase Complex]:::outcome D --> E[Prothrombin → Thrombin]:::action E --> F[Fibrinogen → Fibrin Monomers]:::action F --> G[Fibrin Polymerization]:::action H[Factor XIII Activation<br/>by Thrombin]:::action --> I[Cross-linked Fibrin Clot]:::outcome G --> I E --> H ``` ### The Prothrombinase Complex - **Factor Xa** (activated by either pathway) combines with **Factor Va** (its cofactor) on a phospholipid surface. - This **prothrombinase complex** converts **prothrombin (Factor II)** to **thrombin (Factor IIa)**. - Calcium ions are essential for assembly. ### Fibrin Formation and Stabilization 1. **Thrombin cleaves fibrinogen** → fibrin monomers (A and B fibrinopeptides released). 2. **Fibrin monomers polymerize** → loose fibrin clot (held by hydrogen bonds). 3. **Thrombin activates Factor XIII** → Factor XIIIa (transglutaminase). 4. **Factor XIIa cross-links fibrin** → stable, covalently bonded clot (resistant to fibrinolysis). ### Why Option 3 Is Incorrect **High-Yield:** Both the extrinsic AND intrinsic pathways activate Factor X. The extrinsic pathway (TF + Factor VII) is faster and initiates clotting, but the intrinsic pathway (Factor XIa + Factor IXa + Factor VIIIa in the tenase complex) also activates Factor X and provides sustained amplification. Neither pathway works "alone" — they are **complementary and redundant**. This is why isolated deficiencies of either pathway may have mild or variable bleeding, but combined defects are severe. **Clinical Pearl:** In this patient, normal aPTT and PT suggest both pathways are intact, and the hypercoagulable state is likely due to elevated Factor V or other prothrombotic factors (not shown in the question). ### Correct Statements (Options 0, 1, 2) - **Option 0:** ✓ Correct. Factor V is the essential cofactor for Factor Xa in the prothrombinase complex. - **Option 1:** ✓ Correct. Thrombin is a serine protease that cleaves fibrinogen to release fibrinopeptides and expose polymerization sites. - **Option 2:** ✓ Correct. Factor XIII (fibrin-stabilizing factor) is activated by thrombin and cross-links fibrin via transglutaminase activity, creating a stable clot. [cite:Harrison 21e Ch 139]