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
The most common etiological agent for acute bronchiolitis in infancy is: 
Select and explain. 
A. Influenza virus, B. Para influenza virus, C. Rhinovirus, D. Respiratory syncytial virus
D: Respiratory syncytial virus
Answer: D: Respiratory syncytial virus 
Explanation: Acute bronchiolitis is a serious acute lower respiratory infection in infants affecting children between 1 and 6 months of age. It is most commonly caused by Respiratory syncytial virus. Other causative organisms includes Parainfluenza virus 3, 1 and 2, adenovirus, influenza virus and rarely Mycoplasma pneumonia. Ref: Essential Pediatrics, 6th Edition By O P Ghai, Pages 352-4; Nelson's Essential of Pediatrics, 17th Edition, Pages 1415-16.