A 32-year-old woman presents with a 3-week history of nasal congestion, purulent nasal discharge, and facial pain over the maxillary region. She has already completed a 10-day course of amoxicillin-clavulanate without improvement. On examination, she has tenderness over the maxillary sinuses and purulent discharge in the middle meatus. Plain sinus X-ray shows opacification of the maxillary sinuses bilaterally. What is the most appropriate next step in management?