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
Denominator while calculating the secondary attack rate includes- 
Select and explain. 
A. All the people living in next fifty houses, B. All the close contacts, C. All susceptible among close contacts, D. All susceptible in the whole village
C: All susceptible among close contacts
Answer: C: All susceptible among close contacts 
Explanation: The denominator consists of all persons who are exposed to the case. More specifically the denominator may be restricted only to susceptible contacts if means are available to distinguish the susceptible persons from the immune. (refer pg no 100 park 23rd edition)