35-year-old woman presents to the emergency department complaining of intense, right sided flank pain, nausea, and vomiting for the past 6 hours. Plain film x-rays of the abdomen reveal radiopaque stones that look like antlers obstructing her entire renal pelvis. Laboratory tests reveal a normal serum creatinine and a urine pH of 11.
Which of the following is the most likely chemical composition of these stones?
The correct answer is:
C) Magnesium ammonium phosphate
Magnesium ammonium phosphate (struvite) stones are strongly associated with urinary tract infections with organisms that split urea with their urease enzyme. This results in highly alkaline urine, a key finding associated with magnesium ammonium phosphate stones. These stones may grow rapidly over weeks to months and may get so large that they fill the entire renal pelvis and are called staghorn calculus because they look like antlers. These stones appear radio-opaque.
Answer choice A: Calcium oxalate, is incorrect. Calcium oxalate stones represent the majority of kidney stones. The risk of calcium oxalate stone formation increases as urine oxalate excretion increases. These stones are radiopaque but are usually associated with a low calcium diet, with increased intestinal calcium absorption and with malabsorption syndromes such as inflammatory bowel disease.
Answer choice B: Cystine, is incorrect. Cysteine stones are most commonly found in children and result from the genetic disorder, cystinuria.
Answer choice D: Uric acid, is incorrect. Uric acid stones are radiolucent. Conditions that increase the risk of uric acid stones include gout, chronic diarrhea, diabetes mellitus, and the metabolic syndrome.
Key Learning Point
Struvite stones are made up of magnesium ammonium phosphate and are produced by urea-splitting organisms, resulting in highly alkaline urine.
