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Urinary calculi may form anywhere in the lower
urinary tract (urolithiasis).
the most common sites being the pelvicalyceal system and bladder.
The two main
factors for stone formation are increased concentration of solute in urine (low
throughput or primary increase in metabolite), and reduced solubility of solute
(due to persistently abnormal urinary pH).
Conditions that cause these factors to operate are low fluid intake, urine
urinary tract infection, and primary metabolic disturbances.
The most common urinary stones, accounting for 80% of cases, are composed of
or phosphate. Half of these cases are associated with idiopathic hypercalciuria,
with only about
10% being caused by hypercalcaemia.
Other cases may be caused by hyperoxaluria, which has several associations, e.g.
bowel disease. The second most common type of calculi, accounting for 15% of
are those composed of magnesium, ammonium, and calcium phosphates (struvite).
They are associated with infection in the lower urinary tract as a result of
organisms, which make urine permanently alkaline.
Uric acid stones account for about 5% of cases and are predisposed by conditions
hyperuricaemia, e.g. gout. However, 50% of patients with uric acid stones do not
hyperuricaemia, and it is suggested that production of persistently acid urine
is the predisposing factor.
Cystine stones are rare, accounting for under 1% of cases. They are seen in
tubular transport defects causing cystinuria.
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