A 38-year-old woman presents with a 3-week history of nasal congestion, purulent nasal discharge, and facial pain over the maxillary region. She reports that symptoms began after an upper respiratory tract infection. On examination, she has tenderness over the maxillary sinuses and purulent secretions in the middle meatus. Nasal endoscopy shows edema and pus in the ostiomeatal complex. CT paranasal sinuses reveals opacification of the maxillary sinuses with air-fluid levels. Which of the following is the most appropriate next step in management?