A 57-year-old male is admitted to the hospital with left fla…

A 57-year-old male is admitted to the hospital with left flank pain.  Blood tests indicate hematuria (blood in the urine) and anemia.  An MRI reveals that blood flow in the left renal vein is being occluded by an aneurysm in an artery where the vein crosses the abdominal aorta.  The aneurysm is most likely located in which structure that is located anterior to the left renal vein?