American Board of Internal Medicine (ABIM) Certification Practice Exam

Struvite stones are composed mostly of which substances?

1. Calcium oxalate

2. Magnesium ammonium phosphate

3. Uric acid

4. Cystine

The correct answer is: Magnesium ammonium phosphate

Struvite stones, also known as infection stones, are primarily composed of magnesium ammonium phosphate. This composition arises in the urinary system, particularly in conditions where there are urinary tract infections caused by urea-splitting bacteria such as Proteus species. These bacteria produce an enzyme called urease, which hydrolyzes urea into ammonia and carbon dioxide. The increased concentration of ammonia raises the pH of urine, leading to the precipitation of magnesium and phosphate, resulting in the formation of struvite stones. In contrast, calcium oxalate stones are formed from calcium and oxalate, which is unrelated to the process that generates struvite stones. Uric acid stones are formed from excess uric acid in the urine and are not involved in the same pathological process as struvite stones. Cystine stones are related to a genetic disorder of cystine metabolism, leading to high levels of cystine in the urine. Thus, while each of the other substances mentioned can lead to stone formation, they do not contribute to the creation of struvite stones as magnesium ammonium phosphate does.