**Core Concept**
Staghorn calculi are a type of kidney stone that forms in the renal pelvis and can grow to fill the renal collecting system. They are typically composed of magnesium ammonium phosphate (struvite) and are associated with urinary tract infections (UTIs), particularly those caused by urease-producing bacteria.

**Why the Correct Answer is Right**
Staghorn calculi form as a result of the action of urease-producing bacteria, such as Proteus, Klebsiella, and Pseudomonas, which split urea into ammonia and carbon dioxide. The ammonia then reacts with magnesium and phosphate ions in the urine to form struvite crystals, which aggregate to form the calculus. The struvite crystals have a characteristic "staghorn" shape due to their branching and twig-like morphology.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because staghorn calculi are not typically composed of calcium oxalate monohydrate, which is a common component of other types of kidney stones.
* **Option B:** This option is incorrect because staghorn calculi are not typically associated with cystine stones, which are caused by a genetic disorder that leads to the accumulation of cystine in the urine.
* **Option C:** This option is incorrect because staghorn calculi are not typically composed of uric acid, which is a common component of other types of kidney stones.

**Clinical Pearl / High-Yield Fact**
The presence of a staghorn calculus in the kidney is often associated with a UTI caused by a urease-producing bacteria, and prompt treatment of the infection is essential to prevent further stone growth and potential complications such as kidney damage or sepsis.

**Correct Answer:** D. Struvite