diet and decreased acidity of the urine help promote struvite crystal formation. Urethral plugs in cats are usually composed of struvite crystals and organic matter. Urate stones Urate (C5H4N4O3) stones, usually ammonium urate (NH4·C5H4N4O3) or sodium urate monohydrate (Na·C5H4N4O3×H2O), form in an acidic to neutral urine. They are usually small, yellow-brown, smooth stones. Urate stones form due to an increased excretion of uric acid in the urine. Dalmatians (especially males) and to a lesser extent Bulldogs are genetically predisposed to the formation of urate stones because of an altered metabolism of purines. Dalmatians have a decreased rate of urate hepatic transport, leading to only about 30 to 40 percent conversion of urate to allantoin, compared with greater than 90 percent conversion in other breeds. Dogs with portosystemic shunts or endstage liver disease also have increased uric acid excretion in the urine due to reduced conversion of uric acid to allantoin and ammonia to urea. Urate stones make up about six percent of all stones in the cat. Urate stones can be dissolved using a diet with reduced purines that alkalinizes and dilutes the urine. Allopurinol is used in dogs with altered purine metabolism to prevent the formation of uric acid. Feeding a diet high in purines while simultaneously administering allopurinol can result in the formation of xanthine (C5H4N4O2) stones. Cystine stones Cystine ((SCH2CHNH2COOH)2) stones form in an acidic to neutral urine. They are usually smooth and round. They are caused by increased urine excretion of cystine (a relatively insoluble amino acid) in dogs with a defect in renal tubule reabsorption of cystine. Dietary reduction of protein and alkalinization of the urine may help prevent formation. Medications such as D-penicillamine and 2-MPG contain thiol, which forms a soluble complex with cystine in the urine. Dog breeds possibly predisposed to formation of cystine stones include Bulldogs, Dachshunds, Basset Hounds, Chihuahuas, Yorkshire Terriers, Irish Terriers, and Newfoundlands. In Newfoundlands, cystinuria is inherited as an autosomal recessive trait, but in the other breeds it is a sex linked trait and found primarily in male dogs. Calcium phosphate stones Calcium phosphate, also known as hydroxyapatite (Ca10(PO4)6(OH)2), stones form in neutral to alkaline urine. They are usually smooth and round. Calcium phosphate is usually a component of struvite or calcium oxalate stones and is infrequently a pure stone. They form more readily with hypercalcaemia. Dog breeds possibly predisposed to calcium phosphate stone formation include Yorkshire Terriers,