Cystine stones (uroliths) occur when an inherited defect of the kidneys causes a build up of cystine in the dog's urine. Cystine is an amino acid that is filtered and reabsorbed by the kidneys. The build up of this amino acid in the urine leads to the formation of cystine stones. Certain breeds are more affected than others by cystine urolithiasis. These include English Bulldogs, Newfoundlands, Dachshunds, Chihuahuas, and Mastiffs. Mixed breed dogs with the ancestry of these genetically susceptible breeds can also be affected.Royal Canin