**Core Concept**
Struvite stones are a type of radiolucent renal stone, which means they are not visible on plain X-rays due to their composition. They are formed from magnesium ammonium phosphate and are often associated with urinary tract infections (UTIs) caused by urease-producing bacteria.

**Why the Correct Answer is Right**
Struvite stones are radiolucent because they contain little to no calcium, which is the primary component of radiopaque stones. The magnesium ammonium phosphate composition of struvite stones makes them difficult to visualize on plain radiographs. The presence of these stones is often suspected in patients with recurrent UTIs and symptoms such as flank pain and hematuria.

**Why Each Wrong Option is Incorrect**
**Option A:** Calcium oxalate stones are radiopaque and are the most common type of renal stone. They are visible on plain X-rays due to their high calcium content.
**Option B:** Uric acid stones can be radiolucent, but they are typically less dense and more likely to be seen on plain X-rays than struvite stones. They are often associated with conditions such as gout and leukemia.
**Option C:** Cystine stones are radiolucent but are typically less dense than struvite stones and have a more characteristic appearance on imaging studies.

**Clinical Pearl / High-Yield Fact**
Struvite stones are often associated with UTIs caused by urease-producing bacteria such as Proteus, Klebsiella, and Pseudomonas. Identifying the underlying infection is crucial for successful treatment of struvite stones.

**Correct Answer: C. Cystine stones are radiolucent but are typically less dense than struvite stones and have a more characteristic appearance on imaging studies.