<prompt> 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?Acute EpiglottitisLaryngotracheobronchitisForeign body AspirationRetropharyngeal Abscess  <response>: 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. 