USMLE题库Genitourinary21-30.pptxVIP

Genitourinary 题库 Q 21 A 21-year-old male is being evaluated for recurrent kidney stones. Sodium cyanide is added to his urine, followed by a sodium nitroprusside solution, and the urine becomes red-purple in color. Further quantitative laboratory evaluation is most likely to detect which of the following abnormalities in this patient? Hypercalciuria Hyperoxaluria Hyperuricosuria Hypocitraturia Aminoaciduria A 21 Correct answer:E Recurrent nephrolithiasis in a young patient should alert the physician to the possibility of an inborn error of metabolism. Cystinuria results from a defect in the renal proximal tubules, which causes decreased reabsorption of the amino acid cystine. The increased cystine excretion can precipitate stone formation. The cystine transporter also contributes to the reabsorption of other dibasic amino acids (arginine, ornithine, lysine). Increased excretion of these amino acids does not result in stone formation, however, as these are relatively soluble. Patients with cystinuria experience recurrent kidney stones, beginning at a young age. Hexagonal cystine crystals on urinalysis are pathognomonic. These crystals are relatively radiopaque due to the density of sulfur-containing compounds. They form in acidic urine (pH 6). The detection of cystine in urine is important not only for establishing a diagnosis, but also for monitoring treatment effects and for predicting the rate of stone formation. The sodium cyanide-nitroprusside test described in the vignette detects sulfhydryl groups, and is a rapid qualitative determinant of the presence of urine cystine. The cyanide added to the urine converts cystine to cysteine. The nitroprusside then binds cysteine, causing a purple discoloration in 2-10 minutes. Treatment of cystinuria involves hydration and alkalinization of urine. (Choices A, B, C and D) Hypercalciuria, hyperoxaluria, hyperuricosuria and hypocitraturia are risk factors for recurrent calcium stone formation. However, these abnormaliti