Most common cause of abdominal aoic aneurysm is

. Aoic wall contain smooth muscle cell matrix,elastin(in tunica media),collagen (in adventitia) as safe net in the wall to provide tensile strength prevent aneurysms formation.Elastin in tunica media is degraded and reduced significantly in infrarenal aoa in relation to collagen,abscence or less vasa vasorum in infrarenal aoa and atherosclerotic ubstability of the medial wall of aoa cause infrarenal aoa and more prone to develop aneurysms.Increased proteolytic matrix metalloproteinase(MMP) which is derived from aoic smooth muscle cells and macropgages cause elastin and collagen degradation and increase in the diameter of aneurysms. Refer; SRB's manual of Surgery page no:198.