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
[{'content': 'Earliest event of vascular trauma is? \nSelect and explain. \nA. Vasoconstriction, B. Platelet adhesion, C. Platelet aggregation, D. Vasodilatation', 'role': 'user'}, {'content': 'Answer: A \nExplanation: 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.', 'role': 'assistant'}]