A 70-year-old man has abdominal pain with mass in the abdomen. Angiography reveals aneurysm of the aorta.Most likely cause is -
Clinical  presentation of the abdominal aortic aneurysm

Effects old person (>50 years), male > female.
Most commonly asymptomatic and usually detected on routine examination as palpable, pulsatile non-tender mass.
As size increases pain may occur. Complications
Rupture into the peritoneal cavity and retroperitoneal tissues with massive, potentially fatal haemorrhage. The risk of rupture is directly related to the size of the aneurysm.
Obstruction of iliac, renal, mesenteric or vertebral branches due to direct pressure or narrowing of ostia → ischemic damage to respective organs.
Impingement on adjacent structures e.g. compression of the ureter or vertebral erosion.
Embolism from atheroma or mural thrombus.
Trauma
Atherosclerosis
Syphilis
Congenital