Cystinuria

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

Cystinuria is one of the more common genetic disorders. Its overall prevalence is about 1 in 7,000 in the population.

Cystinuria is the most common defect known in the transport of an amino acid.

Within the body, many molecules are able to pass across the membranes that surround cells. These molecules can accomplish this feat due to specific transport systems. These systems include special receptors on the membrane of the cell and special carrier proteins. The receptor recognizes the molecule and receives it on the cell membrane. Then the molecule hitches a ride through the cell membrane on the back of a carrier protein.

With such remarkable specificity, it is little wonder that sometimes there are defects in transport systems. Several dozen different diseases are now known to be due to transport defects.

Cystinuria is caused by the defective transport of cystine and several other amino acids through the cells of the kidney and the intestinal tract.

Although cystine is not the only overly excreted amino acid in cystinuria, it is the least soluble of all naturally occurring amino acids. Cystine precipitates, or crystallizes out of urine and forms stones (calculi) in the kidney, ureter, bladder, or anywhere in the urinary tract.

The cystine stones (below) compared in size to a quarter (a U.S. $0.25 coin) were obtained from the kidney of a young woman by percutaneous nephrolithotripsy (PNL), a procedure for crushing and removing the dense stubborn stones characteristic of cystinuria.

Source: MedicineNet.com
http://www.medicinenet.com/cystinuria/article.htm