Cystinuria: An inherited (genetic) disorder of the transport of an amino acid (a building block of protein) called cystine resulting in an excess of cystine in the urine (cystinuria) and the formation of cystine stones.

Cystinuria is the most common defect in the transport of an amino acid. Although cystine is not the only overly excreted amino acid in cystinuria, it is the least soluble of all naturally occurring amino acids. Cystine tends to precipitate out of urine and form stones (calculi) in the urinary tract.

Small stones are passed in the urine. However, big stones remain in the kidney (nephrolithiasis) impairing the outflow of urine while medium-size stones make their way from the kidney into the ureter and lodge there further blocking the flow of urine (urinary obstruction).

Obstruction of the urinary tract puts pressure back up on the ureter and kidney. causing the ureter to widen (dilate) and the kidney to be compressed. Obstruction also causes the urine to be stagnant (not moving), an open invitation to repeated urinary tract infection. The pressure on the kidneys and the urinary infections results in damage to the kidneys. The damage can progress to renal insufficiency and end-stage kidney disease, requiring renal dialysis or a transplant.

The stone are responsible for all the signs and symptoms of cystinuria, including:

• Hematuria -- blood in the urine;
• Flank pain -- pain in the side, due to kidney pain;
• Renal colic - intense, cramping pain due to stones in the urinary tract;
• Obstructive uropathy -- urinary tract disease due to obstruction; and
• Urinary tract infections.

There are several genetic types of cystinura. Cystinuria type I is caused by mutation in a gene called the SLC3A1 amino acid transporter gene located on chromosome 2p. Cystinuria type III appears to be due to mutation at a separate gene locus.

For a fuller understanding of cystinuria, including the diagnosis, the different modes of treatment, the genetics of cystinuria and the prognosis for persons with this disease, please see Cystinuria.