What defines an aneurysm?

Defining an aneurysm revolves around understanding the structural integrity of blood vessels. An aneurysm is characterized by a localized dilation or swelling of an artery, which occurs due to a weakness in the vessel wall. This weakening can lead to an abnormal bulging that, if left untreated, poses the risk of rupture, leading to potentially life-threatening hemorrhaging.

In the context of the other options: a complete obstruction of blood flow in an artery indicates a thrombosis, not an aneurysm. A narrowing of an artery due to plaque refers to atherosclerosis, which affects blood flow but does not involve the concept of bulging or dilation associated with aneurysms. Lastly, a temporary surge in blood pressure does not reflect a structural change in the artery itself, unlike an aneurysm, which is a permanent alteration in the artery's size and shape.

Thus, the unique characteristic of an aneurysm as a swelling resulting from wall weakness is clearly presented in the correct answer.