**Core Concept**
The bilateral spider leg sign on intravenous pyelography (IVP) is a characteristic radiological finding associated with a specific renal condition. This sign is caused by the stretching and thinning of the renal calyces due to the expansion of cysts within the kidneys.

**Why the Correct Answer is Right**
Polycystic kidney disease (PKD) is a genetic disorder characterized by the development of numerous cysts filled with fluid within the kidneys. As these cysts grow and expand, they stretch and thin the surrounding renal tissue, including the calyces. On IVP, this stretching and thinning of the calyces gives rise to the characteristic "spider leg" appearance, with the calyces appearing as thin, elongated structures radiating from the renal pelvis. This is a classic radiological finding in PKD, distinguishing it from other renal conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** Hydronephrosis, a condition characterized by the dilation of the renal pelvis and calyces due to an obstruction in the urinary tract, does not typically present with a spider leg sign on IVP. Instead, hydronephrosis is often associated with a "dilated" or "flattened" appearance of the renal pelvis and calyces.
**Option B:** Hypernephroma, a type of renal cell carcinoma, can cause various radiological abnormalities, including a mass or an area of increased density within the kidney. However, it is not typically associated with a spider leg sign on IVP.
**Option C:** Renal stones can cause a range of radiological findings, including a "stone cast" or "staghorn calculus" on IVP. However, they do not typically cause a spider leg sign.

**Clinical Pearl / High-Yield Fact**
In patients with a suspected diagnosis of polycystic kidney disease, a characteristic "spider leg" appearance on IVP can be a valuable diagnostic clue. This sign is often seen in combination with other radiological findings, such as multiple cysts and an enlarged kidney.

✓ Correct Answer: A. Polycystic kidney