**Core Concept**
The patient's presentation of abdominal pain and fusiform dilatation of the descending aorta suggests a condition involving aortic wall degeneration, leading to aneurysm formation. This condition is often associated with atherosclerosis, hypertension, and cystic medial degeneration.

**Why the Correct Answer is Right**
The most likely cause of a fusiform dilatation of the descending aorta is atherosclerotic aneurysm. Atherosclerosis is a chronic inflammatory process involving the deposition of lipids, inflammatory cells, and fibrous elements within the arterial wall. This leads to the destruction of the elastic lamina and the weakening of the aortic wall, resulting in aneurysm formation. The fusiform shape of the aneurysm indicates a circumferential involvement of the aortic wall.

**Why Each Wrong Option is Incorrect**
**Option A:** Dissection of the descending aorta typically presents with a more acute onset of symptoms, such as severe chest or back pain, and a more characteristic "double aortic" sign on imaging. The fusiform dilatation in this case does not suggest a dissection.
**Option B:** Traumatic aneurysms often occur after a significant injury to the aorta, such as a car accident or a penetrating wound. The patient's history does not suggest a traumatic cause for the aneurysm.
**Option C:** Inflammatory aneurysms, such as those caused by giant cell arteritis, typically present with systemic symptoms like fever and weight loss, and often involve the more proximal aorta.

**Clinical Pearl / High-Yield Fact**
Atherosclerotic aneurysms are more common in older adults and are often associated with hypertension, smoking, and atherosclerotic disease in other vascular beds.

**Correct Answer:** C.