Cystine Kidney Stones
Lemon yellow with a sugary coating these form only in people who have an inherited kidney disorder called cystinuria. Although the kidneys function well, they permit abnormal amounts of four amino acids to enter the urine. Three do not matter that we know of. The fourth makes crystals and the cystine kidney stone type.
Because people with cystinuria lose large amounts of cystine in their urine, stones readily grow large, and fast. Stones probably form in the urine itself. But cystine crystals can plug the ends of kidney tubules, as calcium phosphate crystals do, causing cell damage.
Since cystinuria is an inherited disease, stones may begin in childhood. Effective treatment always requires very large amounts of fluids to dilute the urine. The few effective drugs resemble cysteine. Their sulfur groups bond with cysteine to form a ‘mixed disulfide’ more soluble than cystine. But their side effects can limit use.