Step 1: Recall what an antrochoanal polyp is.
An antrochoanal polyp starts inside the maxillary sinus, grows a stalk that passes through the sinus opening, crosses the nasal cavity, and can even hang down into the nasopharynx (throat) through the choana. It is usually a single, benign polyp and does not go away on its own.
Step 2: Decide the treatment goal.
Because the polyp keeps growing and blocks the nose, treatment must remove the whole polyp, including the part sitting inside the maxillary sinus (the antral part). Removing only the visible nasal part without the antral stalk leads to quick recurrence.
Step 3: Choose the surgical approach suited to children.
The classic Caldwell-Luc operation goes through the canine fossa, above the upper teeth, to open the maxillary sinus from outside. In a child this route can injure developing tooth buds and disturb facial bone growth, so it is avoided when possible. Endoscopic intranasal polypectomy removes the polyp and its antral attachment through the nostril under endoscopic guidance, without cutting through the face, and is gentler on a growing skull.
Step 4: Rule out the other options.
(A) Caldwell-Luc operation: Effective in adults but not the first choice in children because of the risk to facial growth and tooth buds.
(C) Corticosteroids: May shrink some inflammatory nasal polyps but cannot remove a large antrochoanal polyp with an antral stalk, and it soon regrows.
(D) Wait and watch: The polyp keeps enlarging and can cause worsening nasal block or aspiration risk if it hangs low, so observation alone is not appropriate.
Step 5: Final Answer.
The most appropriate management in a child is endoscopic intranasal polypectomy with removal of the antral part.
\[ \boxed{\text{Intranasal polypectomy}} \]