A perinephric pseudocyst is correct!
This image shows a very large perinephric pseudocyst. A small kidney with normal architecture is present within the fluid-filled pseudocyst.
In this case, the cat had bilateral pseudocysts and kidney failure, so standard medical management of chronic kidney disease (CKD) was initiated. Not all cats with pseudocysts are in kidney failure. If the abdominal distention becomes so profound as to cause discomfort—as it did in this cat—surgically removing the capsule will allow the fluid to drain into the abdomen where the omentum and peritoneum may be able to reabsorb some or all of the fluid.