What causes croup, which leads to obstruction due to swelling below the vocal cords?

Croup is primarily caused by a viral infection, which leads to inflammation and swelling in the laryngeal area below the vocal cords. This condition is most commonly associated with viruses like parainfluenza, although other viruses such as adenovirus and respiratory syncytial virus (RSV) can also be responsible. The ensuing edema in the airway is what contributes to the characteristic symptoms of croup, including a barking cough, stridor, and hoarseness.

In contrast to other potential causes of airway obstruction, such as bacterial infections or allergic reactions, which tend to present differently or necessitate different treatment approaches, it is the viral nature of croup that is crucial to understanding its underlying pathology. Bacterial infections often cause more severe and acute forms of illnesses but do not typically produce the same viral symptoms that lead to the hallmark presentation of croup. Allergic reactions might cause swelling in the airways but are less common as a direct cause of croup symptoms and would be treated differently. Environmental irritants may exacerbate symptoms but are not the primary instigators of croup itself. Thus, identifying the correct viral origin of croup is essential for appropriate management and treatment.