**Core Concept**
The appearance of "spider leg" on excretory urography is a radiological sign associated with the distortion of the normal renal architecture due to the presence of multiple cysts. This sign is indicative of a specific renal pathology characterized by the growth of numerous cysts within the kidneys.

**Why the Correct Answer is Right**
Polycystic kidney disease (PKD) is a genetic disorder that leads to the formation of numerous fluid-filled cysts within the kidneys. The cysts in PKD are typically filled with a protein-rich fluid and can grow to varying sizes, causing distortion of the normal renal architecture. On excretory urography, the presence of these cysts can cause the renal collecting system to appear elongated and distorted, resembling the appearance of a spider's legs. This is due to the compression of the renal parenchyma by the expanding cysts, which can cause the calyces to appear stretched and elongated.

**Why Each Wrong Option is Incorrect**
**Option A:** Hydronephrosis refers to the dilation of the renal pelvis and calyces due to an obstruction of the urinary tract. While hydronephrosis can cause distortion of the renal collecting system, it does not typically cause the "spider leg" appearance seen in PKD.

**Option B:** Medullary sponge kidney is a congenital disorder characterized by the presence of ectatic collecting ducts within the renal medulla. While it can cause distortion of the renal collecting system, it does not typically cause the "spider leg" appearance seen in PKD.

**Option D:** Renal cell carcinoma is a type of kidney cancer that can cause distortion of the renal architecture. However, it does not typically cause the "spider leg" appearance seen in PKD.

**Clinical Pearl / High-Yield Fact**
The "spider leg" appearance on excretory urography is a classic radiological sign of PKD, and its presence should prompt further evaluation for this condition. It is essential to recognize this sign to diagnose PKD early, as it can lead to the development of complications such as hypertension, kidney failure, and increased risk of hemorrhage.

**✓ Correct Answer: B. Polycystic kidney**