The most appropriate next step in management is to remove the struvite stone in the left renal pelvis. Struvite stones are composed of magnesium ammonium phosphate and occur only when ammonium production is increased, which elevates the urine pH and decreases the solubility of phosphate. This is most commonly a consequence of chronic upper urinary tract infection (UTI) with a urease-producing organism, such as Proteus or Klebsiella. Struvite stones can grow rapidly and become large, filling the entire renal pelvis and taking on a characteristic “staghorn” shape. Although struvite stones affect less than 10% of patients with kidney stones, they occur more commonly in women and in patients predisposed to chronic or recurrent UTI, including those with urologic diversions or neurogenic bladder.

Although treatment of the initial upper UTI is important to prevent struvite stone development, once struvite stones are formed, they are difficult to treat medically, including with chronic antibiotics. Antibiotics may not penetrate the stone, and colonizing bacteria may create an alkaline environment within the stone that promotes continued or recurrent UTI, stone growth, and chronic inflammatory damage to the kidney. Because of this, stone removal is indicated in most cases, and kidney outcomes have been shown to be improved when struvite stones are removed compared with medical therapy. Removal is commonly by percutaneous nephrolithotomy, shock wave lithotripsy, or a combination of both procedures.

Dietary phosphate reduction and urine acidification would be expected to discourage struvite stone formation but are of minimal effectiveness once struvite stones have developed.