Which of the following is the most common cause of acute laryngotracheal bronchitis?

Parainfluenza virus type 1 and 2 are the most common organism causing acute laryngotracheobronchitis. Laryngotracheobronchitis is the most common cause of infectious upper airway obstruction in children between the ages of 6 months and 3 years. Children typically present with several day history of low grade fever, rhinorrhea, sore throat and mild cough. In the next 2-3 days they develop hoarseness and barking seal cough. Lateral view of neck on Xray shows normal epiglottis and supraglottic structures, overdistended hypopharynx, haziness within the subglottis with subglottic narrowing greater on inspiration. AP view of the neck shows steeple sign or pencil point sign Ref: CURRENT Diagnosis & Treatment: Pediatrics, 20th Edition, Chapter 38