**Core Concept**
Aortitis refers to inflammation of the aorta, which can be caused by various conditions such as syphilis, Takayasu arteritis, or giant cell arteritis. The chest X-ray is a crucial diagnostic tool to visualize the aorta and detect any abnormalities.

**Why the Correct Answer is Right**
The correct answer is the "double density sign" or "double contour sign" on the chest X-ray. This sign is caused by the presence of inflammatory tissue surrounding the aorta, which appears as a double density or contour on the radiograph. This is due to the edema and inflammation of the aortic wall, which causes it to appear thicker and more prominent than normal. The double density sign is a classic radiographic feature of aortitis and can be seen in various conditions that cause aortic inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely a distractor that mentions a different radiographic finding, but it is not specific to aortitis. For example, it might mention a "tram-track sign" which is associated with pulmonary edema or other conditions.

**Option B:** This option might mention a "notched aortic knob," which can be seen in conditions such as Marfan syndrome or other aortic diseases, but it is not specific to aortitis.

**Option C:** This option might mention a " mediastinal widening," which can be seen in various conditions, including aortitis, but it is not specific enough to be considered diagnostic of aortitis.

**Clinical Pearl / High-Yield Fact**
The double density sign or double contour sign on chest X-ray is a classic radiographic feature of aortitis, and it is essential to remember this sign when diagnosing conditions that cause aortic inflammation.

**Correct Answer:** C.