The loss of HPRT prevents the body from recycling purines, so the body has to make more purines 'from scratch' to keep up with its needs. Because the purines are not recycled, they get wasted. In the human body, most of the purine wastes are turned into uric acid. This leads to a process where the body keeps making more purines, which keep getting turned into uric acid. This vicious cycle explains why people with Lesch-Nyhan disease make too much uric acid.
Uric acid itself is not bad for the body. The kidneys concentrate most of it into the bladder so it can be eliminated in the urine. But when too much uric acid accumulates, it clumps together to form tiny crystals or stones. These crystals like to form in cool parts of the body, such as the skine or joints of the hands and feet.
The crystals also like to form in the kidneys and bladder, where they are especially concentrated. These stones are most dangerous, as described in the next section.