What type of aneurysm has a sac-like formation?

A saccular aneurysm is characterized by a bulging or sac-like formation on one side of the blood vessel. This type of aneurysm typically arises from a localized weakness in the vessel wall, allowing a portion to protrude outward, creating the distinctive "sac" appearance. Saccular aneurysms are most commonly found in the arteries of the brain, but they can occur in other locations as well.

In contrast, fusiform aneurysms involve a more diffuse dilation of the vessel, affecting a larger segment of the artery in a symmetrical manner without a distinct sac-like formation. Cylindrical and irregular aneurysms also do not exhibit the characteristic sac-like feature found in saccular aneurysms. Hence, the defining feature of the saccular aneurysm is its localized, ballooning shape, making it the correct option for this question.