A 16-year-old boy is brought to the emergency department with sudden onset of left-sided abdominal pain and blood in his urine. The pain waxes and wanes in intensity and does not improve with rest or position changes. He has a lengthy history of similar pain episodes, but this is the first time he has had gross hematuria. Physical examination shows costovertebral angle tenderness on the left side. Microscopic examination of the urine is shown below.
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Further laboratory evaluation is most likely to reveal which of the following abnormalities in this patient?
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Recurrent nephrolithiasis in a young patient should raise suspicion for cystinuria, which is confirmed by the pathognomonic finding of hexagonal-shaped crystals on urinalysis. Cystinuria is an autosomal recessive disorder affecting the high-affinity, sodium-independent dibasic amino acid transporter found on the apical membrane of intestinal and proximal renal tubular epithelial cells. This prevents cystine (cysteine homodimer that forms disulfide bonds) and dibasic amino acids (eg, ornithine, lysine, and arginine) from being reabsorbed in the proximal renal tubules, leading to urine supersaturation with cystine and formation of cystine stones (ornithine, lysine, and arginine are relatively soluble in the urine and do not form stones).
Patients with suspected cystinuria without cystine crystals on urinalysis can be diagnosed by detecting elevated urinary cystine levels (ie, aminoaciduria). The sodium cyanide-nitroprusside test is a qualitative screening test that detects the presence of urinary cystine. Cyanide is initially added to the urine, converting cystine to cysteine. Afterward, nitroprusside is added and reacts with the sulfhydryl group on free cysteine, causing a red-purple discoloration (positive test). Treatment of cystinuria involves increasing hydration and urinary alkalinization (eg, acetazolamide).
(Choices B, C, D, and E) Hypercalciuria (eg, sarcoidosis), hyperoxaluria (eg, Crohn disease), and hypocitraturia (eg, distal renal tubular acidosis) are risk factors for recurrent calcium stone formation; high concentrations of calcium or oxalate promote precipitation into stones, while citrate inhibits stone formation by complexing with calcium in the urine, preventing spontaneous nucleation and crystal growth. Hyperuricosuria (eg, gout) is a risk factor for recurrent uric acid stones, particularly in the setting of low urine pH.
Educational objective:
Cystinuria results from defective cystine reabsorption by proximal renal tubular epithelial cells. It most often presents with recurrent stone formation at a young age. Urinalysis shows pathognomonic hexagonal cystine crystals, and the sodium cyanide-nitroprusside test can be used to detect excess cystine in the urine.