As a card carrying member of the Unilateral Renal Agenesis club, I have to say that it is a peculiar thing - the day you discover that you are essentially mutant. For me it was very late in life - I was 29.
The thing is, if you live a relatively healthy life, and never need x-rays or scans, then no one looks inside you to discover missing organs. In my case, I had applied for life insurance - it seemed prudent. There were some simple tests, one of which was a urine test. So then I get this cryptic note from the insurance company that I was denied and to seek medical attention. No details, no help, nothing. I had to beg and argue and cajole and get downright rude before the company would even let me know which part of the test showed what so I had something to tell my doctor when I went to see him... I am not going to rant about this now.
Anyway, it seems that I had hypertension (high blood pressure) which was causing my kidneys - we thought I had two at the time - to release too much protein into my urine. Which prompted an ultrasound. So I laid on the table and the woman moved the device back and forth for a long time over one kidney without comment or action. Then she switched sides and there was much moving back and forth accompanied by considerable button pushing and switches and activity by her on the machine. I became very concerned as to why this side prompted so much more activity. Then she left.
A bit later she returned with a doctor who proceeded to also observe both sides briefly and then start grilling me about whether I had ever had surgery before, or been in an accident, or ever had someone tell me I only had one kidney... surprise!. The side with the button pushing was her taking pictures cause there was something there - the other had nothing to scan - so I had been worried about the wrong thing completely. Anyway more activity followed - scans because the one kidney was enlarged, etc., etc., etc. There was considerable stress and confusion. (I do not recommend it.)
Anyway, to recap the one of key elements in Rook's fine observations of the condition - 1 kidney can prompt hypertension. From my experience this then puts pressure on the kidney, causing other problems. It is kind of a vicious circle. But some blood pressure pills and diet changes makes the other problems go away, and then you get used to the idea that you only have one of that which everyone else has two. In fact, at this point I couldn't even tell you which kidney I have... I am pretty sure that I am living off of the left one, but I am not positive. My wife knows. It causes her vexation if she dwells on it.