**Core Concept**
Radiolucent renal stones are those that do not contain sufficient calcium or other radiopaque materials to be visible on standard radiographic imaging. This characteristic makes them difficult to diagnose using X-rays alone, requiring alternative imaging modalities like ultrasound, CT scans, or laboratory analysis for detection.

**Why the Correct Answer is Right**
The correct answer is **C. Uric acid stones**, which are commonly radiolucent due to their composition. Uric acid stones form when the urine is too acidic, and they are more likely to occur in individuals with conditions like gout or those on certain medications. Unlike calcium stones, which are radiopaque and can be visualized on plain X-rays, uric acid stones do not contain sufficient calcium to be visible on standard radiographs.

**Why Each Wrong Option is Incorrect**
**Option A:** Calcium oxalate stones are typically radiopaque and can be visualized on X-rays, making them incorrect.
**Option B:** Cystine stones are also radiopaque and can be seen on X-rays, eliminating them as the correct answer.
**Option D:** Struvite stones are composed of magnesium, ammonia, and phosphate, and while they can be radiolucent in some cases, they are not the most common radiolucent renal stone.

**Clinical Pearl / High-Yield Fact**
Remember that radiolucent renal stones often require further investigation, such as ultrasound or CT scans, to confirm their presence and composition.

**Correct Answer:** C. Uric acid stones.