**Core Concept**
Aortitis refers to the inflammation of the aorta, which can be caused by various conditions such as syphilis, Takayasu arteritis, or giant cell arteritis. The inflammation of the aorta can lead to various radiological findings on chest x-ray.

**Why the Correct Answer is Right**
Aortitis typically presents with a dilated aorta, which can be visualized on chest x-ray. The dilatation of the aorta is due to the loss of elasticity and the subsequent dilation of the aortic wall. This can lead to an increase in the diameter of the aorta, which is a common feature of aortitis on chest x-ray. The dilated aorta can be seen as a widened mediastinum on the chest x-ray.

**Why Each Wrong Option is Incorrect**
**Option A:** Aortic dissection is a condition where there is a tear in the intima of the aorta, leading to separation of the intimal and medial layers. This is not a common feature of aortitis on chest x-ray.
**Option B:** Aortic calcification is a feature of atherosclerosis, not aortitis. Aortic calcification can be seen on chest x-ray as a calcified aorta.
**Option C:** Aortic regurgitation is a condition where there is incomplete closure of the aortic valve, leading to regurgitation of blood back into the left ventricle. This is not a feature of aortitis on chest x-ray.

**Clinical Pearl / High-Yield Fact**
Aortitis can be caused by various conditions, including syphilis, Takayasu arteritis, or giant cell arteritis. It is essential to consider these conditions in the differential diagnosis of aortitis, especially in patients with systemic symptoms such as fever, weight loss, or fatigue.

**Correct Answer: C. Dilated aorta.**