A 55-year-old woman comes to the hospital due to fever, chills, left flank pain, and dysuria for 2 days. She was treated for a urinary tract infection twice in the past 3 months, the latest being 4 weeks ago. Both times, the patient was compliant with antibiotic treatment and her symptoms improved. She has no other medical problems and takes no medications. Temperature is 39 C (102.2 F), blood pressure is 110/70 mm Hg, pulse is 90/min, and respirations are 16/min. BMI is 40 kg/m². Examination shows left costovertebral angle tenderness. Laboratory results are as follows: Kidney ultrasound shows a 1.3-cm, irregularly shaped stone in the left renal pelvis. Which of the following is the most likely mechanism of stone formation in this patient?

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11ec25c8_d2b2_e8c2_bc81_3dd3e9920fee_MD0003_00 This patient has a large, irregular kidney stone associated with fever and a history of recurrent urinary tract infections. This presentation suggests a magnesium ammonium phosphate (struvite) stone. Struvite stones are caused by urease-producing organisms (eg, Proteus, Klebsiella). Hydrolysis of urea yields ammonia, which alkalinizes the urine and facilitates the precipitation of struvite crystals. Because of the large quantities of urea excreted in urine, these stones can grow very rapidly and fill the renal calyces (staghorn calculi). The large size of these calculi prevents them from passing into the ureter, so patients typically have symptoms related to associated recurrent infections rather than acute renal colic. If left untreated, the stones can seed further infections, and chronic infection and obstruction can lead to loss of kidney function. Eradication of staghorn calculi is difficult; antibiotics alone do not eliminate the source of the infection because bacteria grow within the stone matrix. Stone removal (eg, percutaneous nephrolithotomy) is typically recommended, and nephrectomy may be required for patients with a chronically infected, nonfunctioning kidney. (Choice A) Hyperparathyroidism is associated with an increased risk for calcium stones; although parathyroid hormone increases renal reabsorption of calcium, the increased release of calcium from bone leads to an increased filtered calcium load and net calcium excretion. In contrast, hypoparathyroidism decreases renal calcium reabsorption, but because of low overall calcium levels, kidney stones are rare. (Choices B and E) Increased production of uric acid (eg, myeloproliferative disorders) can increase the risk of uric acid stones, which form in acidic urine (impaired uric acid reabsorption is very rare). Uric acid stones are typically small, relatively uncommon, and less likely to be seen in the context of recurrent urinary tract infection. (Choices C and D) Dietary oxalate forms complexes with calcium in the intestinal lumen and is eliminated in the feces. If intestinal concentrations of calcium are low (eg, low-calcium diet, fat malabsorption), absorption of free oxalate is increased; the increased oxalate load is filtered by the kidneys and can cause calcium oxalate stones. Although a small oxalate stone may serve as a nidus for the formation of a struvite stone, struvite formation is related primarily to urinary ammonia rather than to calcium and oxalate. Educational objective: Magnesium ammonium phosphate (struvite) causes large kidney stones in patients who have recurrent upper urinary tract infection with urease-producing organisms (eg, Proteus, Klebsiella). Antibiotics alone do not eliminate struvite stones, which can harbor bacteria, leading to further infection. Stone removal is usually required. References: Renal struvite stones-pathogenesis, microbiology, and management strategies. (http://www.ncbi.nlm.nih.gov/pubmed/24818849)

A 55-Year-Old Woman Comes to the Hospital Due to Fever