This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
[{'content': 'The first case introduced into the population group to be studied is called: \nSelect and explain. \nA. Index case, B. Primary case, C. Initial case, D. Reference case', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans: b (Primary case) Ref: Park, 19th ed, p. 90The term primary case refers to the first case of a communicable disease introduced into the population unit being studied.The term index case refers to the first case to come to the attention of the investigator; it is not always the primary case. Secondary cases are those developing from contact with primary case.', 'role': 'assistant'}]