All of the following are true regarding platelet aggregation and the role of von Willebrand factor (vWF) EXCEPT:

Explanation

## von Willebrand Factor and Platelet Aggregation **Key Point:** vWF is a **pro-aggregatory** molecule that **promotes** platelet adhesion and aggregation, NOT inhibits it. vWF acts as a bridge between platelets and subendothelium, and between platelets themselves via fibrinogen. ### Phases of Platelet Hemostasis ```mermaid flowchart TD A[Vessel injury exposes subendothelium]:::outcome --> B[vWF binds to collagen]:::action B --> C[vWF-GPIb interaction: platelet adhesion]:::action C --> D[Platelet activation: shape change, granule release]:::action D --> E[ADP + Thrombin released]:::action E --> F[P2Y12 + PAR activation]:::action F --> G[Fibrinogen bridges platelets: aggregation]:::action G --> H[Platelet plug formation]:::outcome ``` **High-Yield:** vWF has THREE critical roles: 1. **Adhesion** — GPIb-vWF interaction anchors platelets to subendothelium 2. **Bridging** — vWF carries Factor VIII (protects it from degradation) 3. **Aggregation** — vWF binds to GPIIb/IIIa on activated platelets and acts as a bridge ### Correct Statements (Options 1, 2, 4) | Statement | Mechanism | Clinical Relevance | |-----------|-----------|--------------------| | **Option 1** | vWF-GPIb binding mediates initial adhesion to subendothelium | vWF deficiency → bleeding; loss of initial platelet arrest | | **Option 2** | Thrombin (via PAR-1, PAR-4) ↑ intracellular Ca²⁺ → amplifies aggregation | Thrombin is the most potent platelet agonist | | **Option 3** | ADP (P2Y12 receptor) → secondary aggregation; clopidogrel blocks P2Y12 | Dual antiplatelet therapy targets P2Y12 + COX-1 | ### Why Option 4 Is Wrong **Option 4 claims:** "vWF directly binds to fibrinogen and **prevents** platelet-to-platelet bridging, thereby **inhibiting** aggregation." **This is FALSE for two reasons:** 1. **vWF does NOT prevent aggregation** — it **promotes** aggregation. vWF binds to GPIIb/IIIa on activated platelets and acts as a bridge between platelets. 2. **Fibrinogen, NOT vWF, is the primary platelet-to-platelet bridge.** Fibrinogen binds to GPIIb/IIIa on adjacent platelets, forming the platelet plug. vWF can also bind GPIIb/IIIa but is not the primary aggregator. **Clinical Pearl:** In von Willebrand disease, patients have **bleeding** (not excessive clotting) because vWF is deficient or dysfunctional — proving vWF is essential for hemostasis, not inhibitory. [cite:Robbins 10e Ch 4]