A 16-year-old boy presents with progressive unilateral nasal obstruction and epistaxis for 3 months. Anterior rhinoscopy reveals a smooth, glistening, vascular mass in the nasopharynx. Contrast-enhanced CT shows a well-defined mass with intense homogeneous enhancement arising from the sphenoid region, with widening of the pterygopalatine fossa. What is the most appropriate next step in management?