**Core Concept**
The "figure of 8" sign in chest X-ray is a radiological finding that suggests a specific condition involving the aorta. It is a characteristic appearance due to the abnormal curvature of the aortic arch and the descending aorta.

**Why the Correct Answer is Right**
The "figure of 8" sign is often seen in patients with aortic dissection, particularly in the Stanford type A dissection. In this condition, there is a tear in the intimal layer of the aorta, leading to the separation of the intimal and medial layers. This results in the formation of two lumens within the aorta, one true and one false, which can cause the aorta to take on a characteristic "figure of 8" shape on chest X-ray. The aortic arch and descending aorta are involved in this condition, and the "figure of 8" sign is a result of the abnormal curvature of these two parts of the aorta.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the "figure of 8" sign is not typically associated with aortic aneurysm. While aneurysms can cause significant changes in the aorta's shape, the "figure of 8" sign is more characteristic of aortic dissection.
* **Option B:** This option is incorrect because pneumothorax is not associated with the "figure of 8" sign. Pneumothorax would typically result in a different radiological appearance, such as a visible air-leak or a lung collapse.
* **Option C:** This option is incorrect because mediastinal widening is a non-specific finding that can be seen in many conditions, including aortic dissection. However, the "figure of 8" sign is a more specific indicator of aortic dissection.

**Clinical Pearl / High-Yield Fact**
The "figure of 8" sign is a classic radiological finding in aortic dissection, particularly in Stanford type A dissection. However, it is essential to remember that this sign can be subtle and may not always be visible on chest X-ray. A high index of suspicion and careful evaluation of the aortic arch and descending aorta are necessary to make this diagnosis.

**Correct Answer: C. Mediastinal widening is a non-specific finding that can be seen in many conditions, including aortic dissection.