Even within the urology community, it is difficult to "only" operate on kidneys. I was once in the exact same situation you were; I really enjoyed renal physiology and thought it would be cool to operate on them. First off, understand this; there is very little true renal physiology involved in renal surgeon practice (in fact, renal transplant probably has the most medical management involved), so if that is why you want to operate on them, you won't be happy. That said, a wise chief resident I had as a third year caught wind of my plan and told me I needed to see a liver transplant before I committed to the urologist--> kidney transplant surgeon track I was about to begin. From that first split liver transplant, I knew I wasn't a urologist, but a general surgeon. We deal with as much renal physiology as do the urologists (which satisfies that itch I always had), can perform nephrectomies in the face of an emergency/nasty NE tumor or GIST and know what to do when the urologist nicks the IVC or bowel when their (partial) nephrectomy goes bad. I chose the general surgery route to transplant, and now have found something I enjoy even more and I am glad I'm not locked into cystos and prostatectomies for the rest of my career (even though they bill like mad).