Answer: Antrochoanal polyp

Histology: On clinical exam, the lesion was seen as a unilateral polyp that arose in the maxillary antrum and projected through the choana into the posterior aspect of the nasal cavity. Histologically, the specimen consists of a polypoid tissue fragment lined by respiratory epithelium. The stroma is moderately fibrotic, and contains a chronic inflammatory infiltrate of mostly lymphocytes and plasma cells.

Discussion: Sinonasal polyps represent non-neoplastic stromal and epithelial proliferations of uncertain etiology. By far the most common type of sinonasal polyp is the inflammatory polyp. Inflammatory sinonasal polyps tend to involve adults, and they are almost always multiple and bilateral. The presence of a solitary polypoid mass must always raise the clinical suspicion of a neoplastic process such as an inverted papilloma.

One notable exception to the rule that nasal polyps are multiple, bilateral, and occur in adults is the antrochoanal polyp. These are typically solitary lesions that arise from the maxillary sinus and extend via long pedicles to involve the nasal cavity of children (particularly teenage boys). Although they can obstruct the nasopharynx and be clinically confused with a nasopharyngeal tumor (e.g. angiofibroma), involvement of the nasopharynx is secondary as a result of local extension.
The distinction from an inflammatory polyp is mostly clinical. On histologic grounds the antrochoanal polyp is very similar to the inflammatory polyp, except that its stroma tends to more fibrotic and less edematous; and its inflammatory component is contains relatively fewer eosinophils.