**Core Concept**
Bilateral hydronephrosis is a condition characterized by the dilation of both kidneys' collecting systems 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 kidney damage.

**Why the Correct Answer is Right**
The most common cause of bilateral hydronephrosis is a ureteropelvic junction (UPJ) obstruction or a ureteral obstruction. The UPJ is the region where the renal pelvis meets the ureter. When this junction is obstructed, it prevents the normal flow of urine from the kidney to the bladder, leading to hydronephrosis. The kidneys compensate for this obstruction by increasing their blood flow and glomerular filtration rate to try and maintain urine production. However, prolonged obstruction can lead to renal damage and failure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is too vague and does not specify a particular condition or cause of bilateral hydronephrosis.
**Option B:** This option is incorrect because it refers to a condition that typically affects only one kidney, such as a kidney stone or tumor.
**Option C:** This option is incorrect because it is a condition that can cause unilateral hydronephrosis, but not bilateral hydronephrosis.

**Clinical Pearl / High-Yield Fact**
A classic exam trap is to confuse bilateral hydronephrosis with unilateral hydronephrosis. Remember that bilateral hydronephrosis typically requires a more proximal obstruction, such as a UPJ obstruction, while unilateral hydronephrosis can be caused by a more distal obstruction, such as a kidney stone.

**Correct Answer: D. Ureteropelvic junction obstruction**