## **Core Concept**
The "spider leg" deformity in an excretory urogram is associated with a condition that affects the renal collecting system, leading to a characteristic appearance on imaging. This deformity is typically seen in conditions that cause scarring or damage to the renal parenchyma and collecting ducts.

## **Why the Correct Answer is Right**
The correct answer, **C. Polycystic kidney disease (PKD)**, is associated with the "spider leg" deformity due to the presence of multiple cysts within the kidneys. These cysts can distort the normal architecture of the renal collecting system, leading to the characteristic appearance on an excretory urogram. In PKD, the cysts can cause stretching and thinning of the renal parenchyma and can lead to the calyces being stretched around the cysts, giving a "spider leg" or "spider web" appearance.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, although certain conditions can cause renal scarring and deformity, they do not typically result in the characteristic "spider leg" appearance associated with PKD.
- **Option B:** This option is incorrect as it does not specifically relate to the typical causes of a "spider leg" deformity in an excretory urogram.
- **Option D:** This option is incorrect because it does not accurately represent a condition commonly associated with the described radiographic finding.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that polycystic kidney disease (PKD) can lead to various complications, including chronic kidney disease, hypertension, and the development of renal cysts that can cause the described deformity on imaging. The "spider leg" deformity is a classic radiographic finding that can aid in the diagnosis of PKD.

## **Correct Answer:** . Polycystic kidney disease.