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
In an epidemiological study, the first case which comes to the attention of the investigator is - 
Select and explain. 
A. Reference case, B. Index case, C. Primary case, D. Secondary case
B: Index case
Answer: B: Index case 
Explanation: Primary case refers to the first case of a communicable disease introduced into the first unit being studied. Index case is the first case which comes to the attention of the investigator. They may not always be the same. Ref: 25th edition, Park's Textbook of Preventive and Social Medicine, Page no. 103