**Core Concept**
The "snowman sign" on chest radiography refers to a characteristic appearance of the aortic arch and descending aorta on a lateral chest X-ray. This sign is indicative of a specific clinical condition.

**Why the Correct Answer is Right**
The snowman sign is seen in **Dissecting Aortic Aneurysm**. In a dissecting aortic aneurysm, blood enters the intimal layer of the aorta and dissects through the media, creating two lumens. The aortic arch appears as the head of the snowman, while the ascending aorta and the descending aorta appear as the middle and lower segments, respectively. This sign is a classic radiographic finding in this condition.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the snowman sign is not typically associated with a **Pulmonary Embolism**. Pulmonary embolism may present with other radiographic findings such as the Westermark sign or Hampton's hump.

**Option B:** This option is incorrect because the snowman sign is not typically seen in **Aortic Stenosis**. Aortic stenosis may present with radiographic findings such as an enlarged cardiac silhouette or a calcified aortic valve.

**Option C:** This option is incorrect because the snowman sign is not typically associated with **Pneumonia**. Pneumonia may present with radiographic findings such as lobar consolidation or patchy infiltrates.

**Option D:** This option is incorrect because the snowman sign is not typically seen in **Aortic Regurgitation**. Aortic regurgitation may present with radiographic findings such as an enlarged cardiac silhouette or a dilated aortic root.

**Clinical Pearl / High-Yield Fact**
The snowman sign is a classic radiographic finding in dissecting aortic aneurysm, and its recognition is essential for prompt diagnosis and management. This sign should be remembered as a key feature of this condition.

**Correct Answer:** C. Dissecting Aortic Aneurysm