This patient’s symptoms of hematuria and flank pain suggest that she has a urinary tract infection (UTI). Results of her urinalysis, as well as a staghorn calculus shown on the x-ray below, confirm the diagnosis. “Staghorn stones” are commonly composed of ammonium magnesium phosphate (struvite) and are caused by infection by urease-producing microorganisms, including Proteus species (most frequently), Staphylococcus species, Ureaplasma, and Klebsiella species.
Owing to the action of urease, which breaks down urinary urea into ammonia and carbon dioxide, patients infected by the organisms named above have increased ammonia levels. As ammonia combines with water to increase availability of ammonium in the alkaline urine, struvite stone formation occurs. UTI, mild flank pain, or hematuria are characteristic of this condition. Urinary analysis typically reveals alkaline urine pH (>7.0), often with multiple magnesium ammonium phosphate crystals in the urine sediment. The most common cause of community-acquired UTIs is Escherichia coli, which is urease negative; the second most common cause is Staphylococcus saprophyticus, which is urease positive.
The other organisms listed as answer choices–Escherichia coli, Pseudomonas, Salmonella, and Shigella-- do not produce urease. Furthermore, Salmonella and Shigella are exceedingly uncommon organisms in UTIs. Therefore none of these organisms is the likely cause of a staghorn stone composed of ammonium magnesium phosphate (struvite).