## **Core Concept**
The presentation of a unilateral pink nasal mass in a child is highly suggestive of a juvenile nasopharyngeal angiofibroma (JNA), a benign but locally aggressive vascular tumor. This condition predominantly affects adolescent males. Given its vascular nature, preoperative evaluation to assess the tumor's vascular supply and extent is critical.

## **Why the Correct Answer is Right**
The correct answer, **CT Angiography or MRI**, is crucial for delineating the tumor's extent, involvement of surrounding structures, and its vascular supply. This information is vital for surgical planning, including potential embolization to reduce intraoperative bleeding. JNA's rich vascular supply, often from branches of the external carotid artery, makes imaging that can detail this supply essential.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While a biopsy can provide histological confirmation, it's not the most important investigation prior to surgery due to the risk of significant bleeding and because the clinical and imaging features are often diagnostic for JNA.
- **Option B:** Ultrasound might help in assessing some soft tissue masses but is not the best modality for evaluating the extent of a nasal mass or its vascular supply in the context of JNA.
- **Option D:** A plain X-ray might show a nasal mass or some bony changes but does not provide sufficient detail on the tumor's extent or vascular supply.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **embolization** is often considered preoperatively to reduce the vascularity of JNAs, significantly decreasing the risk of intraoperative bleeding. Imaging that can guide this process, like CT angiography, is therefore critical.

## **Correct Answer:** . CT Angiography or MRI.