What could also cause stridor aside from supra or subglottic swelling?

Stridor is a high-pitched wheezing sound resulting from turbulent airflow in the upper airway, typically indicating a significant narrowing or obstruction. Foreign body aspiration is a primary cause of stridor because it can lead to immediate obstruction of the trachea or upper airway, resulting in increased airflow resistance and turbulence. This obstruction can vary in severity depending on the size and location of the foreign body, leading to the characteristic stridulous sound during inspiration.

On the other hand, while bronchitis, pneumonia, and allergic reactions can cause respiratory symptoms, they are less likely to directly produce stridor as they typically involve lower airway issues or systemic reactions. Bronchitis mainly affects the bronchi, leading to wheezing and coughing, whereas pneumonia affects the lung parenchyma and can produce rales or wheezes without causing stridor. Allergic reactions may lead to symptoms like wheezing or difficulty breathing due to bronchospasm or swelling, but these typically originate more in the lower airways rather than causing the upper airway obstruction characteristic of stridor.