This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
[{'content': 'Which of the following is the preformed toxin involved in mechanism of allergic rhinitis ? \nSelect and explain. \nA. Histamine, B. Leukotriene, C. TXA2, D. PGD2', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is 'a' i.e., Histamine", 'role': 'assistant'}]