Melody Craft, age 16, of Wilmington, Del., for her question:

WHAT IS AN ANEURYSM?

An aneurysm is a weakening of the walls of an artery or vein so that it balloons out. 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.

Aneurysms are sometimes named for their shape. Saccular aneurysms are sac shaped or pouch shaped. Fusiform aneurysms look like spindles and cylindroid ones like cylinders.

But often aneurysms are named for the condition they cause. For example, an arteriovenous aneurysm allows a vein and artery to open into one another.

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

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