**Core Concept**
The "spider leg" sign on excretory urography is associated with a specific anatomical structure that becomes visible due to its unique relationship with the renal pelvis and calyces. This sign is indicative of a condition that affects the renal collecting system, leading to characteristic changes in its radiographic appearance.

**Why the Correct Answer is Right**
The "spider leg" sign is typically seen in cases of **Medullary Sponge Kidney (MSK)**. MSK is a congenital disorder characterized by cystic dilatation of the collecting ducts and tubules within the renal medulla. On excretory urography, the dilated collecting ducts appear as thin, linear radiolucent structures radiating from the renal pelvis, resembling the legs of a spider. This occurs because the dilated collecting ducts are filled with fluid and appear as radiolucent spaces on the radiograph, while the renal pelvis and calyces are more opaque due to the presence of contrast material.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not correct because the "spider leg" sign is not typically associated with **renal cysts**, which are usually solitary and do not have the characteristic radiating pattern seen in MSK.
* **Option B:** This option is not correct because **pyelonephritis** typically presents with more diffuse inflammation and scarring of the renal parenchyma, rather than the characteristic radiographic appearance of MSK.
* **Option C:** This option is not correct because **renal cell carcinoma** typically presents as a solid mass or nodule within the renal parenchyma, rather than the characteristic radiating pattern seen in MSK.

**Clinical Pearl / High-Yield Fact**
The "spider leg" sign on excretory urography is a classic radiographic finding in Medullary Sponge Kidney (MSK), which should be considered in the differential diagnosis of patients with recurrent nephrolithiasis or chronic kidney disease. MSK is a congenital disorder that can lead to recurrent kidney stones and chronic kidney disease if left untreated.

**Correct Answer: D. Medullary Sponge Kidney (MSK)**