**Core Concept**
Extracorporeal shock wave lithotripsy (ESWL) is a non-invasive treatment for kidney stones that uses shock waves to break them into smaller pieces. The effectiveness of ESWL depends on the composition and size of the stone. Some stones are more resistant to shock wave fragmentation than others.

**Why the Correct Answer is Right**
Staghorn calculi are difficult to break using ESWL due to their large size and complex shape, which makes them resistant to shock wave fragmentation. These stones often fill the renal pelvis and calyces, and their dense, branched structure makes them harder to target with shock waves. Furthermore, staghorn calculi often contain struvite or magnesium ammonium phosphate, which are harder to break down than the more common calcium oxalate stones.

**Why Each Wrong Option is Incorrect**
**Option A:** Calcium oxalate stones are actually more commonly treated with ESWL and are generally easier to break down than staghorn calculi.
**Option B:** Uric acid stones can be treated with ESWL, but they are not as hard to break down as staghorn calculi.
**Option C:** Cystine stones are rare and can be treated with ESWL, but they are not typically as resistant to shock wave fragmentation as staghorn calculi.

**Clinical Pearl / High-Yield Fact**
Remember that staghorn calculi often require surgical intervention, such as percutaneous nephrolithotomy (PCNL), due to their large size and complex shape.

**Correct Answer: D. Staghorn calculus.**