Free nephrology board questions: Nephrolithiasis

A 17-year-old boy is evaluated for nephrolithiasis. Two weeks ago, CT for right flank pain showed a 4-mm right distal ureteral stone with mild hydronephrosis and a 1.2-cm staghorn calculus in the left kidney. He passed the stone 3 days ago; it is yellow and 3.5 mm long. His paternal grandfather had kidney stones. Potassium is 4.8 mEq/L, total CO₂ 24 mEq/L, creatinine 1.1 mg/dL, and calcium 9.1 mg/dL. Urinalysis shows pH 6.0, 2+ blood, and negative protein, leukocyte esterase, and nitrite. In addition to sending the stone for analysis, which finding would best establish the diagnosis?

A 49-year-old man with obesity, hypertension, and gout undergoes Roux-en-Y gastric bypass. His medications include lisinopril, furosemide, ibuprofen, and glyburide. About 4-5 weeks later he presents with weakness and fatigue; serum creatinine has risen to 3.7 mg/dL from a normal baseline. Urine microscopy shows 1-3 RBCs/hpf, occasional renal tubular epithelial cells, and abundant colorless, bipyramidal (envelope-shaped) crystals; a kidney biopsy shows tubular crystal deposition that is birefringent under polarized light. Which of the following crystals is MOST likely responsible?

A 58-year-old man with gout and metabolic syndrome (obesity, DM, hypertension) presents with recurrent nephrolithiasis. CT shows bilateral small stones that are RADIOLUCENT (not visible on plain X-ray). 24-hour urine: uric acid 750 mg/day (slightly elevated), pH 5.1 (persistently acidic), creatinine clearance 65 mL/min. Uric acid crystals on urinalysis. Stone composition analysis shows 100% uric acid. Which is the MOST effective single intervention to prevent recurrent uric acid stones?