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
A 4 year old boy, with three days history of upper respiratory tract infection presents with stridor, which decreases on lying down. What is the most probable diagnosis? 
Select and explain. 
A. Acute Epiglottitis, B. Laryngotracheobronchitis, C. Foreign body Aspiration, D. Retropharyngeal Abscess
B: Laryngotracheobronchitis
Answer: B: Laryngotracheobronchitis 
Explanation: Prodromal symptoms of upper respiratory tract infection preceding sings of respiratory tract obstruction is characteristic of croup. Ref: Nelson 17th Edition, Page 1405;1406,14; Respiratory Care: Principles And Practice By Dean Hess, Neil MacIntyre, Shelley Mishoe, William Galvin, 2011, Page 1019.