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: Social & Preventive Medicine
[{'content': 'First case to come to notice of physician is: \nSelect and explain. \nA. Primary case, B. Secondary case, C. Index case, D. Referral case', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Index case', 'role': 'assistant'}]