But if overall renal health was declining that would mean both kidneys are in jeopardy by then right? The organ energy consumption reasoning makes sense. I wonder why nature chose kidneys as the organ to double up on though. Thanks for the reply!

Thank you for such a wonderfully detailed response! I guess barring an anomaly on your tests, doctors would never think to look for single sided kidney failure. I don't know enough about kidney diseases, but I thought about this because of a coworker/acquaintance who currently has to undergo dialysis. In laymen's logic, it would seem unlikely BOTH of her kidneys failed at the same time. Which means at some point one kidney was doing the job of two, but nobody noticed.