## **Core Concept**
The question pertains to the radiological appearance of a "phantom calyx" or "amputated calyx" on an intravenous urogram (IVU), which is a diagnostic imaging technique used to visualize the urinary system. This appearance is indicative of a specific pathological condition affecting the renal calyces.

## **Why the Correct Answer is Right**
The correct answer, **C. Pyelonephritis**, is associated with a condition known as "renal papillary necrosis" or "pyelonephritis with papillary necrosis." In this condition, the renal papillae, which are projections of renal tissue into the minor calyces, undergo necrosis. When a papilla sloughs off, it can cause a calyx to appear amputated or as a phantom on IVU because the contrast material does not fill the calyx due to the obstruction or damage.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because while certain conditions can cause abnormalities on IVU, the specific appearance of a "phantom calyx" is not typically associated with the option provided (which is not specified but can be inferred as not related to papillary necrosis or pyelonephritis).
- **Option B:** Similarly, this option does not directly relate to the known causes of a phantom or amputated calyx appearance on IVU. Conditions not directly linked to papillary necrosis or similar destructive processes within the renal parenchyma are less likely to cause this specific finding.
- **Option D:** This option is also incorrect as it does not accurately represent a condition known to cause the described radiological finding.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that "phantom calyx" or "amputated calyx" on IVU is highly suggestive of **renal papillary necrosis**, a condition often associated with **diabetes mellitus**, **analgesic nephropathy**, and other causes of papillary damage. This finding can be a critical clue in the diagnosis of these conditions.

## **Correct Answer:** C. Pyelonephritis