A 17-year-old man comes to the emergency department and complains of intense left flank pain that radiates to the groin. He refers to his symptom as "stone passage," which he has experienced "so many times since childhood." His uncle has the same problem. Urinalysis shows hexagonal crystals. The urinary cyanide nitroprusside test is positive. Which of the following is the most likely cause of this patient's condition?

Question

A 17-year-old man comes to the emergency department and complains of intense left flank pain that radiates to the groin.  He refers to his symptom as "stone passage," which he has experienced "so many times since childhood."  His uncle has the same problem.  Urinalysis shows hexagonal crystals.  The urinary cyanide nitroprusside test is positive.  Which of the following is the most likely cause of this patient's condition?

Quizplus · Accepted Answer

This patient's personal history of recurrent stones since childhood, family history of nephrolithiasis, and typical hexagonal crystals on urinalysis is suggestive of cystinuria. Cystinuria is a group of disorders (with several modes of inheritance) characterized by impaired transport of cystine and the dibasic amino acids ornithine, lysine, and arginine by the brush borders of renal tubular and intestinal epithelial cells. This leads to decreased reabsorption (ie, increased urinary excretion) of cystine, which-unlike ornithine, lysine, and arginine-is poorly soluble in water. As a result, patients form cystine stones, which are hard and can be radiolucent (or less radioopaque than calcium stones). The urinary cyanide-nitroprusside test can detect elevated cystine levels and can help confirm the diagnosis; it is used as a qualitative screening test and is particularly helpful to detect individuals who are homozygous for the mutations causing impaired amino acid transport. (Choices B, C, and D) Primary hyperparathyroidism due to parathyroid adenoma can lead to recurrent stones, but these are typically calcium oxalate stones. Excess uric acid excretion in the urine due to hyperuricemia (eg, gout) causes uric acid stones, and excess intestinal reabsorption of oxalate as a result of intestinal conditions (eg, Crohn disease, bariatric surgery) causes calcium oxalate stones. The constellation of recurrent stones since childhood, positive family history, hexagonal crystals, and positive cyanide-nitroprusside test is more consistent with cystinuria. (Choice E) Infection with urease-producing microorganisms can result in struvite stone formation, but these stones rarely present acutely and are more likely to cause mild flank pain, urinary tract infection, hematuria. Educational objective: Cystinuria is an inherited disease causing recurrent renal stone formation. A personal history of recurrent kidney stones from childhood and a positive family history for nephrolithiasis should raise suspicion for the diagnosis. Urinalysis shows typical hexagonal crystals. The urinary cyanide-nitroprusside test is used as a qualitative screening procedure.

A 17-Year-Old Man Comes to the Emergency Department and Complains