**Core Concept**
Urinary stones can be classified based on their radiopacity or radiolucency on plain X-ray imaging. Radiolucent stones are those that do not show up on X-ray due to their chemical composition, making them difficult to diagnose without further imaging techniques.

**Why the Correct Answer is Right**
Cystine stones are a type of radiolucent stone. They are formed from the precipitation of cystine in the urine, often due to a genetic disorder called cystinuria. Cystine stones are less common than other types of kidney stones, such as calcium oxalate or uric acid stones, and are more likely to be radiolucent because cystine is not easily visible on X-ray. This makes cystine stones challenging to diagnose without further imaging, such as CT scans or ultrasound.

**Why Each Wrong Option is Incorrect**
**Option A:** Calcium oxalate stones are typically radiopaque, meaning they show up well on X-ray due to their high calcium content.
**Option B:** Uric acid stones can be radiolucent, but they are more likely to be visible on X-ray than cystine stones.
**Option C:** Struvite stones are typically radiopaque due to their high magnesium and ammonium content.

**Clinical Pearl / High-Yield Fact**
Cystine stones are more common in patients with cystinuria, a genetic disorder that affects the kidneys' ability to reabsorb cystine. This can lead to recurrent kidney stones and other complications.

**Correct Answer: C. Cystine stones are a type of radiolucent stone.**