Which of the following is MOST commonly involved in retroperitoneal fibrosis?

Retroperitoneal fibrosis is a nonspecific, nonsuppurative inflammation of fibroadipose tissue of unknown cause that produces symptoms by the gradual compression of tubular structure in retroperitoneal space. The most impoant clinical aspect of retroperitoneal fibrosis is that fibrotic process frequently entraps and constricts the ureters thereby causing obstructive uropathy. Aoa and inferior vena cava are other structures affected by retroperitoneal fibrosis, but their involvement is not as frequent as ureters. Ref: Bailey & Love 25/e, Page 1007; Schwaz 9/e, Page 1279