This patient has a large, irregular kidney stone associated with recurrent urinary tract infections (UTIs), a presentation suggesting a struvite (magnesium ammonium phosphate) 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.  Although many patients with nephrolithiasis have stones of mixed composition, some individuals, particularly women, produce pure struvite stones.

Because of the large quantities of urea excreted in urine, these stones can grow very rapidly and fill the renal calyces (staghorn calculi).  Their size prevents these calculi from passing into the ureter, so patients typically have symptoms related to associated recurrent UTIs rather than acute renal colic.

Eradication of staghorn calculi is difficult; antibiotics alone do not eliminate the source of the infection because bacteria grow within the stone matrix.  Definitive treatment typically requires complete stone removal (eg, percutaneous nephrolithotomy) because any residual stone fragments continue to seed infections.  Nephrectomy may be required when recurrent infection and obstruction result in a chronically infected, nonfunctioning kidney.

(Choice A)  Hyperparathyroidism is associated with an increased risk for calcium stones; although parathyroid hormone increases the 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 for 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 seen in recurrent urinary tract infections.

(Choices C and D)  Dietary oxalate forms complexes with calcium in the intestinal lumen and is eliminated in the feces.  If the intestinal calcium concentration is low (eg, low-calcium diet, fat malabsorption), absorption of free oxalate is increased; this increased 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 calcium and oxalate.

Educational objective:
Magnesium ammonium phosphate (struvite) causes large kidney stones in patients who have recurrent upper urinary tract infections with urease-producing organisms (eg, Proteus, Klebsiella).  Antibiotics alone do not eliminate struvite stones, which can harbor bacteria, leading to further infection.  Complete stone removal is usually required.