This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiological Study, Subject name: Social & Preventive Medicine
[{'content': 'First case which comes to knowledge of investigator is - \nSelect and explain. \nA. Reference case, B. Index case, C. Primary case, D. Secondary case', 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans. is 'b' i.e., Index case o Primary case - First case of communicable disease introduced into the population,o Index case - First case to come to attention of the investigator,o It is not always necessary that primary case will be index case: -i) When the first case noticed by the investigator is also the first case of the population - Primary case and index case are the same.ii) When the first case nociced by the investigator is not the first case of the population (primary case) - Index case and primary case are different.o Secondary cases - Cases those developing from contact with primary case.", 'role': 'assistant'}]