**Core Concept**
The "Figure of 8" sign on a chest X-ray is a radiological finding associated with a specific condition affecting the mediastinum. This sign is characterized by an abnormal curvature of the aortic arch and the right inferior pulmonary artery, resembling a figure of 8. It is an important indicator of a particular type of pulmonary artery anomaly.

**Why the Correct Answer is Right**
The "Figure of 8" sign is typically seen in patients with a right aortic arch, which is a congenital anomaly where the aortic arch is located on the right side of the trachea. This condition is often associated with other vascular anomalies, such as a right-sided ductus arteriosus or an aberrant right subclavian artery. The aortic arch and the right inferior pulmonary artery form a characteristic curved shape on the chest X-ray, resembling a figure of 8.

**Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect as the "Figure of 8" sign is not typically associated with a left aortic arch, which is the normal anatomical location of the aortic arch.
- **Option B:** This option is incorrect as the "Figure of 8" sign is not a characteristic finding in patients with pulmonary embolism, which may show other radiological signs such as the Hampton's hump or the Westermark sign.
- **Option D:** This option is incorrect as the "Figure of 8" sign is not typically associated with a mediastinal mass, which may show other radiological signs such as a well-defined mass or an enlarged lymph node.

**Clinical Pearl / High-Yield Fact**
When interpreting a chest X-ray, it is essential to recognize the "Figure of 8" sign as a possible indicator of a right aortic arch, which may be associated with other congenital vascular anomalies. This knowledge can help clinicians to identify patients who may require further investigation or management.

**Correct Answer: C. Right aortic arch.**