Two siblings, ages 2 and 4, experience fever, rhinitis, and pharyngitis that result in laryngotracheobronchitis. Both have a harsh, bark-like cough and hoarseness. Which of the following viruses is the leading cause of their syndrome?

Parainfluenza viruses (genus Respirovirus) (c) are important causes of respiratory diseases in infants and young children. The spectrum of disease caused by these viruses ranges from a mild febrile cold to laryngotracheo-bronchitis (croup), and bronchitis in young children and adults to more serious bronchiolitis or pneumonia in infants. Adenovirus (a), Coxsackievirus B (b), and rhinovirus (d) cause upper respiratory tract infections that range from mild (common cold) to serious (pneumonia, ARDS), but they do not cause croup. Rotavirus (e) is a major cause of diarrheal illness in infants.