A 20-year-old man presents with recurrent bilateral calcium stones since adolescence. Urine microscopy shows hexagonal crystals. 24-hour urine shows elevated cystine excretion. Which screening test is most appropriate for this condition?
- A Serum parathyroid hormone level
- B Urine culture for urease-producing organisms
- C Cyanide nitroprusside test on urine ✓
- D Plasma homocysteine level
Correct answer: C. Cyanide nitroprusside test on urine
Explanation
Cystinuria causes recurrent stones from childhood. Hexagonal crystals on urine microscopy are pathognomonic. The cyanide nitroprusside test is the screening test that detects cystine in urine (turns purple). Option B describes struvite stones. Option A evaluates hyperparathyroidism. Option D screens for homocystinuria, a different condition without renal stones.
Reference: Smith's General Urology, 19th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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