 The purpose of this study is to learn more about a condition called unclassified hyperoxaluria. People with this condition have large amounts of oxalate in their urine, which can cause kidney stones and kidney failure. We do not know what causes the high level of oxalate in the urine. In this study, we will evaluate absorption of a test dose of oxalate taken orally by measuring the amount of the test dose that is eliminated in the urine. We will compare the results of affected patients to healthy volunteers. 