**1. Introduction**

Cystinuria is an inherited metabolic disorder characterised by the abnormal transport of dibasic amino acids, cystine, lysine, ornithine and arginine, in the intestine and proximal renal tubule [1, 2]. In cystinuria patients these amino acids are excreted in excess concentrations in the urine due to a failure of reabsorption in the proximal tubule. Cystine is relatively insoluble; its presence in high concentrations in the urine predisposes to formation of urinary calculi, which are often large and can form staghorn calculi. On urine microscopy cystine

crystals appear as flat hexagonal crystals [2]. The recurrent formation of the cystine stones can lead to development of chronic kidney disease [3]. Ornithine, lysine and arginine are more soluble and therefore their excretion in excess concentrations in the urine does not produce clinical sequelae. There are currently no known clinical consequences of impaired absorption of these four dibasic amino acids within the intestine.
