A 65-year-old man comes to the emergency department due to acute right flank pain. The pain started an hour ago and he has vomited twice. The pain is 10 on a scale of 0-10 in severity, is crampy, and waxes and wanes. The patient has never before had similar pain. He reports passing "sand" occasionally in his urine. Medical history is significant for obesity, hyperlipidemia, hypertension, gout, and claustrophobia. The patient does not use tobacco, alcohol, or illicit drugs. He is afebrile. Blood pressure is 160/100 mm Hg, pulse is 98/min, and respirations are 16/min. BMI is 54 kg/m². The abdomen is obese and bowel sounds are decreased. Mild tenderness is present over the right flank. Urinalysis shows hematuria and a urine pH of 5. Abdominal ultrasonogram shows right-sided hydronephrosis and dilation of the proximal ureter. Which of the following is the most likely cause of this patient's current symptoms?

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11ec25c8_d95b_5155_bc81_b5a948caf74b_MD0003_00 This patient has colicky flank pain, hematuria, and hydronephrosis, consistent with acute ureterolithiasis. In light of the acidic urine and history of gout, this patient's symptoms are most likely due to uric acid stones. Excess urinary excretion of uric acid in the setting of low urine pH can cause supersaturation of urine and the formation of uric acid crystals. The stones are radiolucent and typically not visible on x-ray but can be seen on CT scan. Ultrasound is useful in detecting large stones but less sensitive for ureteral or small stones, although, depending on stone location and extent of possible obstruction, ureteral dilation or hydronephrosis may be seen. Patients often report passage of small crystals ("sand") in the urine, but this is not a specific symptom. Risk factors for uric acid stones include gout, obesity, diabetes mellitus/metabolic syndrome, chronic diarrhea (due to intestinal bicarbonate loss and compensatory acidification of the urine), and increased systemic uric acid production (eg, myeloproliferative disorders, hemolytic anemia). Alkalinization of the urine with potassium citrate or potassium bicarbonate effectively dissolves the stones, and interventional efforts to clear the stones (eg, ureteroscopic extraction) are usually unnecessary. (Choice A) Malabsorption of fat (eg, Crohn disease) leads to increased binding of calcium by fat in the intestinal lumen, which causes less calcium to be available to form insoluble oxalate complexes, facilitating increased intestinal absorption of oxalate. Renal clearance of the excess oxalate leads to calcium oxalate kidney stones. However, decreased oxalate absorption would not increase the risk. (Choices B and C) Increased release of calcium and phosphorus from bone in hyperparathyroidism can increase the risk of calcium oxalate or calcium phosphate stones. Although tubular reabsorption of calcium is increased, net urinary calcium excretion is increased due to the excess filtered calcium load. However, this patient does not have other symptoms of hyperparathyroidism (eg, fatigue, bone pain), and his acidic urine and risk factors (eg, gout, obesity) suggest uric acid stones. (Choice D) Hydrolysis of urea yields ammonia, which alkalinizes the urine and facilitates the precipitation of magnesium ammonium phosphate (struvite) crystals; recurrent upper urinary tract infection with urease-producing organisms (eg, Proteus, Klebsiella) can cause struvite stones. Because of the large quantities of urea in urine, these stones can grow rapidly and fill the renal calyces (staghorn calculi). This patient has acidic urine and small stones that are unlikely to be composed of struvite. Educational objective: Excess urinary excretion of uric acid and low urine pH can lead to supersaturation of urine and the formation of uric acid stones. Risk factors include gout, obesity, diabetes mellitus/metabolic syndrome, chronic diarrhea (due to acidification of the urine), and increased systemic uric acid production. Alkalization of the urine with potassium citrate effectively dissolves the stones.

A 65-Year-Old Man Comes to the Emergency Department Due to Acute