This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Autacoids, Subject name: Pharmacology
[{'content': 'Low doses of aspirin prolong bleeding time by selec tively inhibiting synthesis of the following mediator in the platelets \nSelect and explain. \nA. Thromboxane A2, B. 5-Hydroxytryptamine, C. Platelet activating factor, D. Prostacyclin', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Antiplatelet aggregation:- Mech of action:- Aspirin inhibits COX irreversibly and thus inhibits synthesis of PGs. NSAIDs inhibit synthesis of both proaggregatory (TXA2- synthesized in platelets) and anti- aggregatory (PGI2- synthesized by endothelium) prostanoids, but effect on platelet TXA2 predominates . At therapeutic doses most NSAIDs inhibit platelet aggregation, bleeding time is prolonged. Therefore aspirin exes anti- thrombotic effect for several days. Risk of surgical and anticoagulant associated bleeding is enhanced. These drugs should be stopped a week before any elective major surgeries to reduce the risk of bleeding. Ref:- kd tripathi; pg num:-194', 'role': 'assistant'}]