**Core Concept**
Staghorn calculus is a type of kidney stone that forms in the renal pelvis and extends into the calyces, resembling the antlers of an elk or stag. It is typically composed of struvite (magnesium ammonium phosphate) crystals, which are formed in alkaline urine.

**Why the Correct Answer is Right**
Staghorn calculi are often associated with urinary tract infections (UTIs) caused by urease-producing bacteria such as Proteus, Klebsiella, and Pseudomonas. These bacteria produce the enzyme urease, which breaks down urea into ammonia and carbon dioxide, increasing the pH of the urine and creating an environment conducive to the formation of struvite crystals. The struvite crystals then aggregate and grow into a staghorn calculus.

**Why Each Wrong Option is Incorrect**
**Option A:** Uric acid stones - Uric acid stones are typically associated with acidic urine and are not usually found in the renal pelvis.
**Option B:** Calcium oxalate stones - While calcium oxalate stones are the most common type of kidney stone, they are not typically found in the renal pelvis and are not associated with staghorn calculi.
**Option C:** Cystine stones - Cystine stones are rare and are associated with cystinuria, a genetic disorder that affects the kidneys and intestines.

**Clinical Pearl / High-Yield Fact**
Staghorn calculi are often associated with urinary tract infections and may cause obstructive symptoms, such as flank pain and hematuria. Prompt diagnosis and treatment of the underlying infection are essential to prevent complications.

**Correct Answer:** D. Struvite crystals.