Which of the following is the most common trigger for asthma exacerbations?

Viral upper respiratory infections (URI) are indeed the most common trigger for asthma exacerbations. This is largely due to the fact that respiratory viruses, especially rhinoviruses, can induce inflammation of the airways, leading to increased bronchial hyperreactivity and airway obstruction. Many individuals with asthma experience worsening of their symptoms during or after a viral URI, as these infections can cause swelling and increased mucus production in the airways.

While environmental allergens, exercise, and pollutants can trigger asthma symptoms, they are typically not as frequent as viral infections in precipitating acute exacerbations. Allergens might lead to symptoms particularly in highly sensitized individuals, but the widespread nature of viral infections makes them a more common trigger across a larger population. Exercise-induced bronchoconstriction is also notable, but it generally affects only those who are already predisposed to it, while URIs affect a broader range of individuals. Pollutants can exacerbate asthma symptoms, but again, not as commonly as the immediate effects seen with viral infections. Thus, viral URIs are recognized as a significant factor in asthma worsening, contributing to their status as the most common trigger for exacerbations.