**Core Concept**
Nasopharyngeal angiofibroma is a highly vascular, benign tumor that primarily affects adolescent males. The investigation of choice should ideally provide a detailed assessment of the tumor's vascularity and extent of involvement.

**Why the Correct Answer is Right**
The investigation of choice for nasopharyngeal angiofibroma is **MRI**. This is because MRI provides excellent soft-tissue resolution, allowing for precise visualization of the tumor's extent and its relationship with surrounding structures. Additionally, MRI can demonstrate the tumor's vascularity through the use of contrast agents, which is crucial for preoperative planning. The high spatial resolution of MRI also enables accurate assessment of the tumor's invasion into surrounding structures, such as the nasal cavity, sinuses, and skull base.

**Why Each Wrong Option is Incorrect**
**Option A:** **CT Scan**. While CT scans can provide useful information about the tumor's extent and bony involvement, they are not as sensitive as MRI for detecting soft-tissue details and tumor vascularity.
**Option B:** **Angiography**. Angiography is useful for identifying the tumor's blood supply and can be used for preoperative embolization. However, it does not provide a comprehensive assessment of the tumor's extent and surrounding structures.
**Option C:** **Ultrasound**. Ultrasound is not a suitable imaging modality for nasopharyngeal angiofibroma due to its inability to provide detailed information about the tumor's extent and vascularity.

**Clinical Pearl / High-Yield Fact**
Nasopharyngeal angiofibroma is a highly vascular tumor that is often associated with significant bleeding during surgery. Preoperative embolization can help reduce blood loss during surgery.

**Correct Answer: C. Ultrasound**