A 45-year-old man is evaluated in the emergency department for severe right flank pain. He is tossing in the bed from the pain. The patient has a history of hypertension, hyperlipidemia, and diet-controlled type 2 diabetes mellitus. BMI is 43 kg/m². Abdominal radiographs show no abnormalities, but CT scan shows a 5-mm stone in the right ureter and several smaller stones in both renal pelvises. Urinalysis results are as follows: The patient subsequently passes a stone, and analysis shows it to be composed of 100% uric acid. Besides high fluid intake, which of the following would be most helpful for this patient to prevent recurrent stones?

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11ec25c8_d4f5_fbc8_bc81_436d846c2fda_MD0003_00 There are 3 likely possibilities when a patient has symptoms consistent with typical renal colic but no stones are identified on conventional radiographs: Radiolucent stones (uric acid stones, xanthine stones)Small calcium stones (<1-3 mm in diameter)Nonstone ureteral obstruction (eg, blood clot, tumor) Radiolucent stones (uric acid stones, xanthine stones) Small calcium stones (<1-3 mm in diameter) Nonstone ureteral obstruction (eg, blood clot, tumor) This patient has uric acid stones, which account for 5%-8% of adult nephrolithiasis cases. These are most commonly seen in patients with unusually low urine pH (often resulting from a defect in renal ammonia excretion) and hyperuricosuria. Major risk factors include gout, diabetes mellitus/metabolic syndrome, increased uric acid production (eg, myeloproliferative disorders, hemolysis), and chronic diarrhea (from intestinal bicarbonate loss and acidification of the urine). Uric acid stones are radiolucent but can often be seen on renal ultrasound or CT scan. Treatment includes hydration, alkalization of the urine, and a low-purine diet. Alkalization of the urine to pH 6.0-6.5 with oral potassium citrate is recommended because uric acid stones are highly soluble in alkaline urine. In addition, citrate is a stone inhibitor and reduces crystallization. Allopurinol can be added if symptoms recur despite initial measures, especially if hyperuricosuria or hyperuricemia occurs. (Choice A) A calcium-restricted diet is not beneficial in the management of uric acid stones. It is not advised even in patients with calcium stones because it can lead to a negative calcium balance and hyperoxaluria from the resulting increase in gastrointestinal absorption of oxalate. (Choice B) Furosemide increases urinary calcium excretion, and the resulting hypercalciuria can increase the risk of calcium stone formation. (Choice C) A purine-restricted, not a high-protein, diet is indicated in patients with uric acid stones secondary to hyperuricosuria. (Choice D) Hydrochlorothiazide decreases urinary calcium excretion and is used in the management of recurrent hypercalciuric renal stones, which are radiopaque and easily visualized on x-ray. Although thiazide diuretics decrease uric acid excretion, they can lower urine pH and increase the risk of uric acid stones. Educational objective: Uric acid stones are often radiolucent but may be seen on renal ultrasound or CT scan. They are highly soluble in alkaline urine; alkalization of the urine to pH 6.0-6.5 with oral potassium citrate is the treatment of choice. References: Uric acid stones and hyperuricosuria. (http://www.ncbi.nlm.nih.gov/pubmed/23089277)

A 45-Year-Old Man Is Evaluated in the Emergency Department for Severe