**Core Concept**
The presentation of unilateral nasal blockade, a mass in the cheek, and epistaxis in a 15-year-old boy suggests a condition involving the nasal cavity and surrounding structures, likely related to a vascular anomaly or a tumor.

**Why the Correct Answer is Right**
The likely diagnosis is a juvenile nasopharyngeal angiofibroma (JNA), a rare, highly vascular tumor that typically affects adolescent males. JNA arises from the nasopharyngeal mucosa and grows rapidly, invading surrounding structures such as the nasal cavity, sinuses, and orbit. The tumor is rich in vascular channels, which can lead to severe nasal bleeding (epistaxis) and nasal obstruction.

**Why Each Wrong Option is Incorrect**
**Option A:** While a nasal polyp can cause nasal blockade and epistaxis, it is unlikely to present as a mass in the cheek.
**Option B:** A mucocele is a mucus-containing cyst that can cause nasal blockade, but it is not typically associated with a mass in the cheek or severe epistaxis.
**Option C:** A nasal fracture can cause nasal blockade, but it is not typically associated with a mass in the cheek or severe epistaxis.

**Clinical Pearl / High-Yield Fact**
Juvenile nasopharyngeal angiofibroma is a highly vascular tumor that can be life-threatening due to massive bleeding, so prompt diagnosis and treatment are essential.

**Correct Answer:** D. Juvenile nasopharyngeal angiofibroma.