Brad Holcomb, age 12, of Milwaukee, Wisc., for his question:

WHAT CAUSES AN ANEURYSM?

An aneurysm is bulging or ballooning of an artery or vein. It is caused by a weakening of the blood vessel.

Aneurysms can form anywhere in the vessels but they are found most commonly in the blood vessels of the brain and in the aorta, the main vessel for arterial blood.

Doctors can operate to replace the aneurysm with a section of aorta that has been preserved or with a plastic tube.

Perhaps the most painful of all is the dissecting aneurysm. This type happens when a patch of hardened material on the blood vessel wall gives away, leaving openings between layers of the vessel's wall. Blood flows between these layers and separates them.