**Core Concept**
The "spider leg" deformity of calyces on intravenous pyelography (IVP) refers to a radiographic finding where the renal calyces appear to be stretched and elongated, resembling the legs of a spider. This deformity is typically associated with a specific type of kidney disease or anatomical abnormality.

**Why the Correct Answer is Right**
The spider leg deformity is characteristic of medullary sponge kidney (MSK), a congenital disorder where the renal medulla is cystically dilated, leading to abnormal calyceal morphology. MSK is caused by the failure of the collecting ducts to develop properly during embryogenesis, resulting in ectatic and dilated calyces that are prone to stone formation and infection. The renal cortex and medulla are usually spared, but the calyces appear stretched and elongated due to the cystic dilatation.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the characteristic radiographic finding of MSK is the spider leg deformity, not a "butterfly" or "bow-tie" appearance.
* **Option B:** This option is incorrect because the "tram-track" sign is associated with a different type of kidney disease, such as nephrocalcinosis or medullary nephrocalcinosis, where calcium deposits cause a characteristic tram-track appearance on IVP.
* **Option C:** This option is incorrect because the "double-density" sign is associated with a different type of kidney disease, such as pyelonephritis or papillary necrosis, where the calyces appear to be filled with a denser material.
* **Option D:** This option is incorrect because the "corkscrew" deformity is associated with a different type of kidney disease, such as chronic pyelonephritis or reflux nephropathy, where the calyces appear twisted and deformed.

**Clinical Pearl / High-Yield Fact**
The spider leg deformity of calyces on IVP is a classic radiographic finding of medullary sponge kidney, which should be considered in patients presenting with recurrent kidney stones, infections, or other symptoms suggestive of this condition.

**Correct Answer:** C.