## **Core Concept**
The presence of an extra ureter on one side, as detected by ultrasound, suggests a congenital anomaly of the urinary tract. This condition is often associated with **duplicated collecting systems** or **ureteral duplication**, which can occur due to various embryological developmental issues.

## **Why the Correct Answer is Right**
The correct answer, **C. Ureteral duplication**, is the most likely cause because it directly explains the presence of a second ureter on the right side. Ureteral duplication is a congenital condition where there are two ureters draining a single kidney or a duplex kidney. This condition arises during embryological development when two separate ureteric buds form from the Wolffian duct, leading to partial or complete duplication of the ureter. This can sometimes lead to complications such as obstruction, infection, or stone formation, which might be related to the patient's symptoms.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is not provided, but typically, incorrect options might include conditions like ureteral stricture, which does not explain the presence of an extra ureter.
- **Option B:** Similarly, this option is not provided, but it could potentially include conditions like urinary tract infection, which does not directly cause the presence of an extra ureter.
- **Option D:** This option is also not provided, but it might include conditions such as a ureterocele, which is a cystic dilation of the terminal ureter and does not directly explain the presence of an extra ureter.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **complete ureteral duplication** is often associated with the **Weigert-Rink** or **ectopic ureter** where one ureter may be obstructed or ectopically inserted, potentially leading to complications. This condition can sometimes be detected incidentally or when complications arise, as in the case of this patient with a kidney stone.

## **Correct Answer:** C. Ureteral duplication.