Magnesium ammonium phosphate stones account for 10 - 15% of urinary tract stones. They are unusual in that they often occur in alkaline, infected urine whereas other urinary tract stones tend to arise in sterile, acid urine.
Triple stones tend to be white and chalky, and may be quite soft. They are associated with urea splitting bacteria such as Proteus and some Staphylococci which convert urea to ammonia, so alkalinizing the urine and precipitating magnesium ammonium phosphate.
As urea is plentiful in urine, these stones may grow to a large size and fill the renal pelvis - staghorn calculus.
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