**Core Concept**
Intravenous urography (IVU) is a radiographic technique used to visualize the urinary system. A 'flower vase' appearance on IVU refers to a specific radiographic pattern that indicates a particular condition affecting the urinary tract.

**Why the Correct Answer is Right**
The 'flower vase' appearance is typically seen in hydronephrosis, which is the dilation of the renal pelvis and calyces due to an obstruction in the urinary tract. This obstruction prevents the normal flow of urine from the kidneys to the bladder, leading to a buildup of urine and subsequent dilation of the renal collecting system. The 'flower vase' appearance is caused by the contrast material accumulating in the dilated renal pelvis and calyces, giving a characteristic pattern that resembles a flower vase.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the 'flower vase' appearance on IVU. The 'flower vase' appearance is specific to hydronephrosis, not to other conditions such as pyelonephritis or renal calculi.
* **Option B:** This option is incorrect because the 'flower vase' appearance is not due to a mass effect in the renal parenchyma. The appearance is caused by the dilation of the renal pelvis and calyces, not by a mass lesion.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the 'flower vase' appearance on IVU is a classic sign of hydronephrosis, which requires prompt medical attention to prevent long-term kidney damage.

**Correct Answer:** C. Hydronephrosis