Simple kidney cyst

A simple kidney cyst is a pocket of fluid that forms in the kidney. Doctors also call it a simple renal cyst, since "renal" just means "having to do with the kidney," and a cyst is just that: a closed sac with fluid inside. Picture a blister on your skin, a little thin-walled bubble full of fluid, except this one bulges out from the surface of the kidney. There can be one or several, and it's the most common kind of cyst found in that organ.

So where does it come from? Honestly, nobody knows the exact cause; what we have are possible explanations. The kidney does its work with nephrons, the tiny filters where urine is made, and each nephron has a little tube that the urine moves along. One stretch of that tube is called the distal tubule. One idea is that the cyst starts in a diverticulum, a small pouch that bulges outward from the wall of that stretch. The other points to the basement membrane, a thin layer that cells sit on like a foundation. With age it goes through involution, meaning it gradually wears down, and if that's combined with some urinary obstruction, meaning urine doesn't flow through completely freely, a cyst could form from there.

And here's the important part: it usually causes no symptoms and doesn't affect kidney function, meaning it doesn't get in the way of the work the kidney does. Because there's no discomfort to tip you off, it's usually found by chance. That happens on an imaging test, a scan that lets doctors see inside the body, done for some other reason.