CYSTINURIA is an uncommon, easily diagnosable, and treatable disease. The latter two points make it a matter of interest to the practicing physician. We may define the disorder from the point of view of the nephron and from that of the patient. Present concepts hold that a congenital defect of kidney function prevents proper reabsorption by the proximal tubular epithelium of cystine, lysine, ornithine, and arginine. The solubility of cystine is such that crystallization and stone formation may occur. Other nephron functions, such as glomerular filtration, tubular reabsorption, and secretion of various solutes are intact.
Clinically, a young person is seen with recurrent, radiopaque stones and no other problems. The diagnosis may be made in several ways, including microscopic examination of urine. It is this point whence the title of this discussion takes its name. Cystine crystals are hexagonal. Virtually no other urine crystal is so shaped. Second, crystals are