**Question:** Staghorn calculus is made out of: March 2004

**Core Concept:** Staghorn calculus refers to a type of kidney stone that involves the entire renal pelvis and calyces, where multiple calculi (stones) merge together. Kidney stones can be composed of various substances, including calcium, oxalate, phosphate, uric acid, and cystine.

**Why the Correct Answer is Right:** Staghorn calculi are typically composed of calcium oxalate or calcium phosphate, in combination with other minerals (e.g., uric acid, cystine, or carbonate apatite). These stones form when the urinary environment becomes supersaturated with the stone-forming substances, leading to crystal nucleation, growth, and aggregation.

**Why Each Wrong Option is Incorrect:**

A. Calcium phosphate stones are less common in staghorn calculi and are usually associated with uric acid stones.
B. Staghorn calculi are not typically formed from uric acid, which is more commonly associated with gout or hyperuricemia.
C. Cystine stones are rare and usually bilateral, while staghorn calculi are usually unilateral.
D. Carbonate apatite stones are less common and often associated with hypercalciuria, not staghorn calculi.

**Clinical Pearl:** The correct understanding of kidney stone composition is essential in determining the underlying etiology, guiding clinical management, and predicting recurrence risks. Staghorn calculi are typically associated with urinary tract infections (UTIs), hypercalciuria, and obstruction.

**Correct Answer:** D. Staghorn calculi are composed primarily of calcium oxalate or calcium phosphate, often mixed with other minerals. These stones develop from the supersaturation of urine with stone-forming substances, leading to crystal nucleation, growth, and aggregation.