This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bleeding Disorders, Subject name: Pathology
Earliest event of vascular trauma is? 
Select and explain. 
A. Vasoconstriction, B. Platelet adhesion, C. Platelet aggregation, D. Vasodilatation
A: Vasoconstriction
Answer: A: Vasoconstriction 
Explanation: Ans. (a) Vasoconstriction(Ref: Robbins 9th/pg 661)Endothelial Injury causes vasoconstriction Q (due to endothelin, serotonin) immediately and markedly reduces blood flow to the injured area.