A 48-year-old woman with a 12-week history of nasal obstruction, thick nasal discharge, postnasal drip, and facial pressure is diagnosed with chronic rhinosinusitis (CRS) without nasal polyps. Nasal endoscopy shows mucopurulent discharge from the middle meatus. She has failed 4 weeks of topical nasal corticosteroid spray. What is the drug of choice for medical management at this stage?