A 62-year-old man is discovered to have hyperlipidemia on screening tests after a routine physical examination. Laboratory studies show total serum cholesterol of 285 mg/dL, LDL of 215 mg/dL, HDL of 38 mg/dL, and triglycerides of 300 mg/ dL. This patient is most at risk of developing an aneurysm in which of the following anatomic locations?