An antrochoanal polyp is associated most commonly with
Antrochoanal polyp arises from the mucosa of the maxillary antrum near its accessory ostium, comes out of it and grows in the choana and nasal cavity. 
Hypertrophied middle turbinate, one of the important differential diagnosis, which is differentiated by its pink appearance and hard feel of the bone on probe testing. 
Antrochoanal polyps are most commonly associated with middle meatus.
Middle meatus
Sphenoethmoidal recess
Inferior meatus
Superior meatus