007’s Q

Q is a general surgeon whose hobby is adapting gadgets for his intricate surgeries. In fact, his reputation as a creative sort spreads far and wide and he often is invited to speak at national meetings. To impress, he frequently appears at these meetings driving his Aston Martin with an ejector seat and license plate, 007.  One particular Sunday, after morning rounds, he drives to a local meeting of residents to demonstrate his idea for imaging retroperitoneal organs.  He swallows a pill that hones in specifically to retroperitoneal organs, then sends an ultrasound beacon to a detector (could work!). Which of the labeled structures will be identified by this device?

Question: Which structure indicated is within the retroperitoneal space?