Posterior urethral valves (PUV) is a condition found only in boys. PUV occurs in one in every 8000 boys born. It affects the urethra (the tube which runs from the bladder to the outside). In boys with PUV, the urethra has a blockage in it near the bladder. This makes it difficult for them to pass urine. As the bladder pushes hard to get the urine out, it causes pressure which may result in urine being pushed back from the bladder into the ureters and kidneys (reflux). This causes the kidneys and bladder to swell. This also may lead to kidney damage.