Abdominal aortic aneurysm: A balloon-like swelling in the wall of the aorta within the abdomen. This swelling weakens the aorta's wall and, because of the great volume of blood flowing under high pressure in the aorta, it can rupture. An abdominal aortic aneurysm is monitored by ultrasound. Surgery is often recommended if the aneurysm is more than 5.5 centimeters (2.2 inches) in diameter or if a smaller aneurysm is enlarging with unusual rapidity.
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