What is B.U.R.?

Apparently my step-daughter's sister has been diagnosed with B.U.R., and neither of us have heard of this disease, nor can we find anything online by doing searches or visiting WebMD.com. It's some kind of renal dysfunction. Can anyone please tell us what this is? We're guessing the R stands for renal, but we have no idea what the B and U stand for. Thanks!