Time to play! : the answer
To review the post with the question, click here
Most of the posted replies were correct! Well done!
This cat had bilateral (only showed the right side) peri-renal pseudocysts. This condition tends to occur in older cats. It appears as a sub-capsular fluid collection (transudate). The sac is made of fibrous connective tissue (usually not a secreting epithelium). The etiology remains unknown, at times obstruction of the lymphatic drainage has been shown.
It is important to evaluate the kidneys as concurrent nephropathy (acute, chronic or neoplastic) is often present.
In this cat, an underlying chronic nephropathy was diagnosed (high BUN and creatinine, small irregular kidneys with poor cortico-medullary distinction). After unsuccessful drainage of the fluid (recollection within 24 hours), surgical resection of the capsules was performed.