Final answer:
The most common cause of laryngotracheobronchitis (croup) is a viral infection, particularly the parainfluenza virus.
Step-by-step explanation:
The most common cause of laryngotracheobronchitis (croup) is a viral infection, particularly the parainfluenza virus. Other viruses that can cause croup include influenza A and B, respiratory syncytial virus (RSV), and adenovirus. These viruses infect the upper respiratory tract, causing inflammation and swelling of the larynx, trachea, and bronchi. This inflammation leads to the characteristic symptoms of croup, such as a barking cough, hoarseness, and difficulty breathing.