BOUT three fourths-mf-all kidney stones arexorn-A posed o f e l c i u m oxaiate'; most calcium oxalate stones also contain a small ambunt of hvdroxva~atite. I I and 10 to 12 percent contain some uric&.'-Ten-20 perwnt.ol~stonas-conta~n%tmyite (magnesium ammonium phosphate) produced by a urinary tract infection with bacteri; that express the enzyme urease? F~e~p e r~e~-~f +~t~~~~~~~~~~~i~~c-. a 1 :~ 4 e-nt 'P contain-more~-rhan~QQ~c~ccnt-hydroxy-apatl or calcium monoh.y-drogen phosphate (brushite), and less than d-pe~~&e..%emp&gfTs~&e. Whereas most calcium oxalate s t 0 5 are less than 2 cm in diameter. struvite, uric acid, and cystine stones may fill the renal collecting system (staghorn calculi). Tiny flecks of calcium salts that encrust calyces can make kidney radiographs seem like pictures of the night sky (nephrocalcinosis). Calcium oxalate, uric acid, or cystine crystallites can scour the urinary tract, cause pain and bleeding, and then dispersea process commonly described as "passing gravel" and more formally termed "crystalluria." The-composition of every stone should be analyzed. Polarization microscopy4 is an inexpensive guide to differentiation; infrared and x-ray diffraction techniques surpass microscopy in precision and sensitivity: but we have not found their use essential. Galoiurn exalate and-calcium phosphate stones are black, gray, or white; on x-ray films they are small (< 1 cm in diameter), dense, opaque, and sharply circumscribed. Uaic.aeid stones are white or orange, and uric acid gravel is orange but nearly transparent radiograph;cally-unless mixed with calcium crystah or struvite. Uric acid stones are typically seen as filling defects on intravenous pyelograms. C T scanning can distinguish them from kidney tissue or blood clots and reveal their sizes and shapes6 Struvite stones seem gnarled and laminated on radiographs; they look like ginger root. Cystine stones are greenish yellow and flecked with shiny crystallites, like mica. On x-ray films, they look like homogeneous pieces of sculpted wax or soap. Urinalysis can reveal the presence of crystals and provide From the S c c t h of Ncphrobgy. Uninrriy of Qligo, Rin);cr S c h d of Mdicinc. 5841 S. Marylud A u. MC 5100. C h i g o. LL60637. w h nprin~ fcquescs should be ddFCgCd I O Dr. COC. Suppated in puc by gmm (POI-33949 ud DK-3349) fran the Narrml LRaitva d Halth. clues to the type of stone; some of the common types are shown in Figure 1.

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We conclude that urinary pH is inversely related to body weight among patients with stones. The results confirm the previously proposed scheme that obesity may sometimes cause uric acid nephrolithiasis by producing excessively acid urine due to insulin resistance.

Stone CaP% has risen for three decades. CaP SF, particularly with brushite stones, receive more ESWL treatments than CaOx SF, not explained by stone number or duration of stone disease. Urine supersaturations explain the high CaP%. High CaP% does not hamper medical stone prevention.