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: Pediatrics
Pertusis affects which age – 
Select and explain. 
A. 2–3, B. <5, C. 5–7, D. > 10 years
B: <5
Answer: B: <5 
Explanation: In prevaccine era, and in countries where immunization is limited the peak incidence of pertussis is in children 1-5 years of age, infant accounts for < 15% of cases.
In recent years, where vaccination is employed, approximately one half of cases have occurred in infants younger than 1 year of age and one fourth in adolescent and adults.