Which composition is typical of struvite renal calculi?

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Struvite stones form in the setting of infection with urease-producing bacteria. Urease breaks down urea into ammonia, alkalinizing the urine, which promotes precipitation of magnesium ammonium phosphate. The typical composition is magnesium ammonium phosphate hexahydrate, and these stones often grow large and branch into a staghorn shape. This infection-driven, alkaline-urine mechanism distinguishes them from other common stones—uric acid stones form in acidic urine, calcium oxalate stones are the most common overall and not infection-related, and cystine stones arise from a genetic disorder. Understanding the urease-positive infection link and the magnesium ammonium phosphate composition is why this option is correct.