Radiography of the following renal stones shows opacity, EXCEPT:
**Core Concept**
Calcium stones, struvite stones, and cystine stones are radiopaque due to their high density. However, uric acid stones are radiolucent because of their low density, making them difficult to detect on plain radiographs.
**Why the Correct Answer is Right**
Uric acid stones are radiolucent because they have a low atomic number and density, which makes them invisible on plain radiographs. This is in contrast to other types of kidney stones, such as calcium stones, which are highly radiopaque. The low density of uric acid stones is due to their composition, which consists of uric acid crystals that do not absorb X-rays well.
**Why Each Wrong Option is Incorrect**
**Option A:** Calcium stones are highly radiopaque due to their high density, making them easily visible on plain radiographs.
**Option B:** Struvite stones are also radiopaque due to their high density, which is a result of their composition of magnesium ammonium phosphate.
**Option C:** Cystine stones are radiopaque due to their high density, which makes them visible on plain radiographs.
**Clinical Pearl / High-Yield Fact**
Uric acid stones are often associated with conditions such as gout, diabetes, and obesity, and are more common in patients with a low urine pH.
**Correct Answer:** D. Uric acid stones.