**Core Concept**
Antrochoanal polyps are benign, unilateral growths originating from the maxillary sinus mucosa, extending through the sphenopalatine foramen into the nasal cavity. Their growth is often obstructive, causing nasal congestion and other respiratory symptoms.

**Why the Correct Answer is Right**
Antrochoanal polyps are characterized by their slow growth, which can lead to significant nasal obstruction and respiratory symptoms. These polyps are composed of mucous glands, fibrous tissue, and inflammatory cells, which cause them to become enlarged. The main symptom is nasal obstruction, often accompanied by a unilateral nasal discharge or bleeding. The polyps typically arise from the middle meatus of the nasal cavity, specifically from the mucosa of the maxillary sinus. The CT scan findings in this case likely show a unilateral, smooth, and well-defined mass extending from the maxillary sinus into the nasal cavity.

**Why Each Wrong Option is Incorrect**
**Option A:** Juvenile Nasopharyngeal Angiofibroma is a highly vascular tumor that typically presents in adolescent males with nasal obstruction, epistaxis, and a mass in the nasopharynx, often extending into the nasal cavity or sinuses. It is not the most likely diagnosis in a 16-year-old girl.
**Option C:** Nasopharyngeal carcinoma is a malignant tumor that typically presents in adults with symptoms such as hearing loss, tinnitus, nasal obstruction, and neck masses. It is not the most likely diagnosis in a 16-year-old girl.
**Option D:** Adenoid hypertrophy typically presents in young children with nasal obstruction, mouth breathing, and middle ear effusion. It is not the most likely diagnosis in a 16-year-old girl.

**Clinical Pearl / High-Yield Fact**
Antrochoanal polyps are often unilateral and can be easily mistaken for other nasal masses. A thorough history and physical examination, along with imaging studies, are essential for accurate diagnosis. A key differentiating feature is the smooth, well-defined mass extending from the maxillary sinus into the nasal cavity.

**✓ Correct Answer: B. Antrochoanal polyp**