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
First case which comes to the notice of investigator is: 
Select and explain. 
A. Index case, B. Initial case, C. Primary case, D. Secondary case
A: Index case
Answer: A: Index case 
Explanation: Ans. (a) Index caseRef: K. Park 23rded. 195* Index Case: In epidemiology, first case of a disease in a group which is seen by an investigator or which brought to the attention of the clinician. It is not always the primary case.* Primary case: first case of communicable disease introduced into the population unit being studied.* Secondary case: all case who develop from primary case* Incubation period: entry of organism till 1st sign/ symptom* Median incubation: time period for 50% case to occur