<prompt> Most common cause of abdominal aoic aneurysm isAtherosclerosisTraumaSyphilisVasculities  <response>: . Aoic wall contain smooth muscle cell matrix,elastinin 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 metalloproteinaseMMP which is derived from aoic smooth muscle cells and macropgages cause elastin and collagen degradation and increase in the diameter of aneurysms. Refer; SRBs manual of Surgery page no:198. 