Complex sentence: Croup (or laryngotracheobronchitis) is triggered by an acute viral infection of the upper airway, leading to swelling and the classical symptoms of a "barking" cough, stridor, and hoarseness.
Split into simpler sentences: Croup (or laryngotracheobronchitis) is a respiratory condition that is usually triggered by an acute viral infection of the upper airway. The infection leads to swelling inside the throat, which interferes with normal breathing and produces the classical symptoms of a "barking" cough, stridor, and hoarseness.