**Core Concept**
Renal calculi, or kidney stones, are solid masses of mineral and organic material formed in the urinary system. The formation of kidney stones is influenced by various factors, including the concentration of ions in urine, pH levels, and the presence of certain substances that can facilitate stone formation.

**Why the Correct Answer is Right**
Uric acid stones are formed in acidic urine, which has a pH below 5.5. They are composed of uric acid, which is a product of purine metabolism. Extracorporeal shock wave lithotripsy (ESWL) is a common treatment for kidney stones, but it is less effective for uric acid stones. This is because ESWL relies on the stone being composed of a material that can be effectively fragmented by shock waves. Uric acid stones are more resistant to fragmentation due to their chemical properties and the way they form in the urinary system. **✓**

**Why Each Wrong Option is Incorrect**
**Option A:** Cystine stones form in acidic urine, which is true. Cystine stones are a type of stone that forms in people with cystinuria, a genetic disorder that affects the kidneys and intestines. They are more likely to form in acidic urine due to the way cystine is metabolized in the body.
**Option B:** Struvite stones form in alkaline urine, which is also true. Struvite stones are a type of stone that forms in people with urinary tract infections (UTIs) caused by certain types of bacteria. These bacteria produce enzymes that raise the pH of the urine, creating an environment conducive to struvite stone formation.
**Option C:** Oxalate stones are radiopaque, which is true. Oxalate stones are one of the most common types of kidney stones and are composed of calcium oxalate. They are visible on X-rays due to their high calcium content, which makes them radiopaque.

**Clinical Pearl / High-Yield Fact**
When it comes to kidney stones, it's essential to remember that the type of stone and the underlying cause can significantly impact treatment options. For example, uric acid stones may require medical management with medications that alter urine pH or increase urine volume, rather than ESWL.

**Correct Answer Line**
✓ Correct Answer: D. Uric acid stones are resistant to ESWL