What structures of the cat urinary system are "retroperitoneal"?

The correct answer is kidneys.

The urinary system of cats consists of the kidneys which are located retroperitoneal (lie behind the peritoneum), the ureters (connect the kidneys to the bladder), the bladder, and the urethra (the tube through which urine exits the body). The functions of the urinary system are to excrete the waste products, to maintain the balance of water and electrolytes (salts) within the cells, to produce hormones erythropoietin and renin (maintaining blood pressure and producing blood cells).