Related Self Assessment Question A 40-year-old paraplegic with an indwelling urinary catheter has recurrent infections. The UA always shows leukocytes and the urine pH is 8. KUB shows the presence of staghorn calculi. What is the most likely type of stone that occurs in this clinical situation? A. Calcium phosphate B. Calcium oxalate C. Cystine D. Struvite E. Uric acid F. Xanthine G. Bilirubin EXPLANATION: Uric acid stones are associated with low urine pH, owing to decreased NH3 production by the kidney. Uric acid is underexcreted in an acid urine. These stones are commonly radiolucent on plain film or IVP but are easily visualized on CT. They often dissolve within weeks if the urine is alkalinized (eg, with potassium citrate). Struvite stones are found if the urine is infected with organisms (especially Proteus species) that produce the enzyme urease. Urease splits urea to CO2 and ammonium; the latter produces the characteristic alkaline urine (urine pH usually 8). The ammonium combines with urinary magnesium and phosphate to form the insoluble struvite (magnesium ammonium phosphate).These stones are opaque and are often large, filling the collecting system (staghorn calculi). Calcium containing stones cause over 70% of kidney stones and are radiopaque. Hypercalciuria and hyperoxaluria (as can be seen in intestinal malabsorption) are contributing factors. Hexagonal urinary crystals are found in cystinuria, an uncommon hereditary disease that starts early in life and if untreated progresses to end-stage renal disease. These stones may be lucent or opaque. Xanthine stones are rare. The pigment calcium bilirubinate causes gallstones, not kidney stones. The answer is D.
