A 42 year old paraplegic woman has a neurogenic bladder and requires an indwelling urinary catheter. She develops a urinary tract infection and is seen by a urologist, who orders abdominal x-rays and an intravenous pyelogram. The radiographic studies demonstrate a very large stone that fills and follows the contours of the renal pelvis. The stone is most likely composed of which of the following?

Staghorn calculi, such as described in the question, are associated with infection by urea-splitting bacteria (notably Proteus species) and are composed of magnesium ammonium phosphate (struvite). They are more common in women than men, and are far more common in patients requiring chronic bladder catheterisation. Calcium-containing stones are typically seen in patients with hypercalciuria without hypercalcemia; one-fifth of patients with this type of stone have hyperuricosuria. Cholesterol is found in gallstones, not kidney stones. Cystine stones are rare; they may be seen in patients with cystinuria, an autosomal recessive disorder of amino acid metabolism. Ref: Wyatt C., Butterwoh IV J.F., Moos P.J., Mackey D.C., Brown T.G. (2008). Chapter 16. Pathology of the Kidney and Bladder. In C. Wyatt, J.F. Butterwoh IV, P.J. Moos, D.C. Mackey, T.G. Brown (Eds), Pathology: The Big Picture.