Urine samples will be collected after the operation at 6 hours, 12 hours, 24 hours and 72 hours. The urine samples will be collected from your/your child's urinary catheter if one is in place for your/your child's regular hospital care, or by urine collected in a cup if you/your child do not have a urinary catheter. This study requires one blood samples of about 1 teaspoon, before surgery. The blood sample will be collected when other blood samples are drawn, to avoid extra needle sticks for blood. The extra blood collected, totaling about 1 teaspoon, is a safe amount for children over 2 years old.