## Core Concept
The ureters are muscular tubes that carry urine from the kidneys to the urinary bladder. They have three natural constrictions where stones can potentially get stuck.

## Why the Correct Answer is Right
The three natural constrictions of the ureters are:
- At the **ureteropelvic junction (UPJ)**, where the renal pelvis transitions into the ureter.
- At the **crossing of the external iliac artery**, where the ureter passes under the iliac vessels.
- At the **ureterovesical junction (UVJ)**, where the ureter enters the bladder wall.

## Why Each Wrong Option is Incorrect
- **Option A:** The ureteropelvic junction is indeed a site of ureteric constriction.
- **Option B:** The crossing of the external iliac artery is also a site of ureteric constriction.
- **Option D:** The ureterovesical junction is another site of ureteric constriction.

## Why Option C is Correct
- **Option C:** The **hilum of the kidney** is not typically listed as a site of ureteric constriction. The hilum is where the renal vessels, nerves, and the ureter enter and exit the kidney, but it is not a point of natural narrowing of the ureter itself.

## Clinical Pearl / High-Yield Fact
It's crucial to remember the three natural constrictions of the ureters because these are common sites where kidney stones can become lodged, causing obstructive uropathy.

## Correct Answer: C. hilum of kidney