Guy, а 17-yeаr-оld bоy whо hаs had a cough for 2 months presents with a worsening cough and green-colored sputum. He has a history of allergic rhinitis. His parents are worried as the cough is not resolving. On examination, his throat is erythematous, the chest is clear on auscultation, and he has hypertrophied lower turbinates bilaterally. The frontal and maxillary sinuses are nontender. What is the cause of his persistent cough?