This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunization, Subject name: Pediatrics
Zero dose of polio vaccine is given: 
Select and explain. 
A. Before giving DPT, B. At birth, C. When child is having diarrhea, D. When child is having polio
B: At birth
Answer: B: At birth 
Explanation: b. At birth(Ref: Nelson's 20/e p 1561, Ghai 8/e p 190-194)OPV zero dose: At birth or as early as possible within the first 15 days.