**Core Concept**
Bilateral hydroureteronephrosis refers to the dilation of both ureters and the renal pelvis due to an obstruction in the urinary tract. This condition can lead to renal impairment and requires prompt diagnosis and treatment. The underlying principle involves the buildup of urine in the upper urinary tract due to a blockage.

**Why the Correct Answer is Right**
The most common cause of bilateral hydroureteronephrosis is a ureteric junction obstruction (UJO), which occurs at the junction of the renal pelvis and the ureter. This obstruction can be due to a congenital anomaly, such as a ureteropelvic junction (UPJ) obstruction, or an acquired condition, such as a tumor or a stone. The obstruction leads to a buildup of urine in the renal pelvis and calyces, causing dilation and potentially leading to renal impairment.

**Why Each Wrong Option is Incorrect**
* **Option A:** While a kidney stone can cause unilateral hydroureteronephrosis, it is less likely to cause bilateral obstruction.
* **Option B:** A pelvic mass may cause unilateral or bilateral obstruction, but it is not the most common cause of bilateral hydroureteronephrosis.
* **Option D:** A retroperitoneal fibrosis can cause ureteral obstruction, but it is a less common cause of bilateral hydroureteronephrosis compared to a ureteric junction obstruction.

**Clinical Pearl / High-Yield Fact**
A key clinical feature of ureteric junction obstruction is the presence of a palpable renal mass, which is often a sign of chronic obstruction. This is due to the renal tissue becoming fibrotic and forming a mass.

**Correct Answer:** C