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 that comes to notice of physician is? \nSelect and explain. \nA. Primary case, B. Secondary case, C. Index case, D. Refer case', 'role': 'user'}, {'content': 'Answer: C \nExplanation: ANSWER: (C) Index caseREF: Park 20th ed p. 91Primary case: first case introduced into the population unit being studiedSecondary case : those developing from contact with the primary caseIndex case : the first case that comes to attention of investigator/ physician', 'role': 'assistant'}]