## **Core Concept**
A staghorn calculus, also known as a staghorn stone, is a type of kidney stone that takes the shape of the renal pelvis and calyces, resembling the antlers of an elk or stag. The composition of staghorn calculi is often related to urinary tract infections (UTIs) caused by specific bacteria.

## **Why the Correct Answer is Right**
The correct answer, **struvite**, is associated with infections caused by **urease-producing bacteria** such as *Proteus*, *Pseudomonas*, and *Klebsiella*. These bacteria split urea into ammonia and carbon dioxide, increasing the pH of the urine, which leads to the formation of struvite stones. Struvite stones are composed of magnesium ammonium phosphate and are typically seen in the context of UTIs, making them a common cause of staghorn calculi.

## **Why Each Wrong Option is Incorrect**
* **Option A:** Cystine stones are associated with **cystinuria**, a genetic disorder leading to excessive excretion of cystine in the urine. While cystine stones can be large, they are not typically staghorn in shape and are less common than struvite stones in this context.
* **Option B:** Calcium oxalate stones are the most common type of kidney stone but are not typically associated with infections or the formation of staghorn calculi.
* **Option D:** Uric acid stones form in acidic urine and are associated with conditions like gout. They are not typically linked to UTIs or the formation of staghorn stones.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that staghorn calculi are often **infection stones** (struvite stones) associated with urease-producing bacteria. Their presence suggests an underlying UTI, which needs to be treated concurrently with stone management.

## **Correct Answer:** . Struvite