Abdominal Spelunking

Freddy, a third-year medical student, is a spelunker extraordinaire, or so he thinks. All he ever talks about is cave diving, how small a hole he can traverse, how deep he can go, how long he can stay underground without sunlight and so on and so on. While helping to harvest tissue from a male donor for transplantation, he drives the chief resident batty, until she finally sends him away to “find a hole and crawl into it”. But then on second thought she issues an alternative challenge to assess his knowledge of the retroperitoneal structure.

Question: “Okay fella, which indicated structure exits the abdomen by traversing an opening?”