(C) Ammonium urate[?]Ammonium Acid Urate Stones:Ammonium Acid Urate Stones represent less than 1% of all stones.Endemic ammonium acid urate urolithiasis is still observed because it comprises bladder calculi in children.Characteristic stone formation in laxative abuse:Laxative abuse acts a factor in kidney stone formation.Laxative abuse causes potassium loss:-As a compensation mechanism kidney produces large amount of ammonium.-Resulting in formation of uncommon stone type - ammonium acid urate.Conditions associated with ammonium acid urate crystallization include laxative abuse, recurrent urinary tract infection, recurrent uric acid stone formation, & inflammatory bowel disease.Those with laxative abuse are strongly encouraged to develop a healthier bowel regimen.Pathogenesis:Urealysis provides an Alkaline Urine Environment & sufficient concentrations of carbonate & ammonia to induce the formation of infection stones.Because urease is not present in sterile human urine, infection with urease-producing bacteria is a pre-requisite for the formation of infection stones.Urea is hydrolyzed to ammonia and carbon dioxide by bacterial urease:-(NH2 )2 CO+H2 O-NH3 +CO2 Alkaline urine that results from this reaction (pH 7.2 to 8.0) favors the formation of ammonium:-NH3 +H2 ONH4+ +OH-pK=9.0TYPES OF RENAL CALCULUS:[?]Oxalate Calculus (Calcium Oxalate):Majority are Oxalate stones, are irregular with sharp projections.Calcium oxalate monohydrate stone is hard & radiodense.[?]Phosphate Calculus:Calcium phosphate often with ammonium magnesium phosphate (Struvite) is smooth & dirty white.It grows in alkaline urine, especially when urea-splitting Proteus organisms are present.Calculus enlarges to fill most of the collecting system, forming a stag-horn calculus.Large stag-horn calculus may be asymptomatic for years, until it presents with haematuria, urinary infection or renal failure.[?]Uric Acid & Urate Calculi:Hard, smooth, multiple & multifaceted.Pure uric acid stones are radiolucent.CT scan differentiates from other causes of filling defects including tumours of the ureter.Most uric acid stones contain some calcium & radio-opaque.[?]Cystine calculus:Uncommon, hereditary error of metabolism leads to cystinuria.Multiple, grow to form to cast of the collecting system, very hard & radio-opaque.