A 35-year-old man with a 2-year history of nasal obstruction and rhinorrhea presents with bilateral nasal masses. Endoscopy reveals smooth, pale, gelatinous polyps arising from the lateral nasal wall. A 28-year-old woman with cystic fibrosis presents with similar nasal obstruction and is found to have bilateral nasal polyps on examination. Which feature best distinguishes antrochoanal polyps from cystic fibrosis-associated nasal polyps?