In the cardiac intervention theatre, during the passage of an intravenous catheter, numerous endothelial cells are dislodged from the lining of the popliteal vein. What substance allows platelet adhesion to the exposed collagen fibers?
**Core Concept**
The question tests the understanding of the initial step in the formation of a hemostatic plug, specifically the role of von Willebrand factor (vWF) in platelet adhesion to exposed collagen fibers during vascular injury.
**Why the Correct Answer is Right**
Von Willebrand factor is a large, multimeric glycoprotein that plays a crucial role in primary hemostasis. When endothelial cells are damaged, vWF is released from Weibel-Palade bodies and binds to exposed collagen fibers, providing a site for platelet adhesion. This is the first step in the formation of a hemostatic plug, facilitating platelet aggregation and subsequent clot formation. The interaction between vWF and collagen is mediated by the A1 domain of vWF, which binds to the collagen fibers.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because platelet-activating factor (PAF) is a phospholipid that activates platelets, but it does not facilitate platelet adhesion to collagen fibers. PAF acts through a specific receptor on platelets, leading to platelet activation, aggregation, and secretion of granular contents.
**Option B:** This option is incorrect because thrombin is a serine protease that converts fibrinogen to fibrin, forming the clot. While thrombin does play a role in the coagulation cascade, it is not directly involved in platelet adhesion to collagen fibers.
**Option C:** This option is incorrect because platelet factor 4 (PF4) is a chemokine stored in platelet alpha-granules that promotes platelet aggregation and inhibits heparin activity. PF4 does not facilitate platelet adhesion to collagen fibers.
**Clinical Pearl / High-Yield Fact**
Remember that von Willebrand disease (vWD) is the most common hereditary bleeding disorder, resulting from a deficiency or dysfunction of vWF. Patients with vWD often present with mucocutaneous bleeding, including easy bruising, nosebleeds, and gastrointestinal bleeding.
**Correct Answer: C. Von Willebrand factor.**