Which of the following can act as a stimulus for bronchial hyperresponsiveness?

Viral upper respiratory infections are a well-known trigger for bronchial hyperresponsiveness. During these infections, the airways can become inflamed and hyperreactive due to the immune response to the virus, leading to increased sensitivity to various stimuli, which can exacerbate asthma symptoms. In patients with asthma, a viral infection can cause the airway smooth muscle to react more intensely, resulting in wheezing, coughing, and difficulty breathing.

This heightened response is a key aspect of asthma pathophysiology; hence, understanding that viral illnesses can contribute significantly to increased bronchial hyperresponsiveness is essential for managing asthma. The other options, while they may have some effects on respiratory health, do not have the same direct association with triggering bronchial hyperresponsiveness as viral infections do.