## **Core Concept**
Radiolucent renal stones are those that do not show up well on plain X-rays because they do not contain materials that are opaque to X-rays, such as calcium. The composition of these stones can include uric acid, cystine, and certain other substances.

## **Why the Correct Answer is Right**
The correct answer involves identifying which of the listed substances are known to form radiolucent stones. Uric acid stones are a classic example of radiolucent stones because they do not contain calcium. Cystine stones, although less common, are also radiolucent. Xanthine stones are another rare type of radiolucent stone.

## **Why Each Wrong Option is Incorrect**
- **Option B:** Calcium phosphate stones are typically radiopaque because they contain calcium, which makes them visible on plain X-rays.
- The other options (a, c, d) are correct components of radiolucent stones.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while uric acid stones can be radiolucent, they often can be seen on CT scans. Also, uric acid stones are associated with low urine pH and can be treated with urinary alkalinization.

## **Correct Answer:** C. uric acid, cysteine, xanthine.