**Core Concept**
Unilateral hydronephrosis refers to the swelling of one kidney due to a blockage in the urinary tract, leading to the accumulation of urine and subsequent dilation of the renal pelvis and calyces.

**Why the Correct Answer is Right**
The most common cause of unilateral hydronephrosis is a ureteropelvic junction (UPJ) obstruction. This obstruction occurs at the junction where the renal pelvis meets the ureter, preventing the normal flow of urine into the ureter. As a result, the kidney becomes distended with urine, causing hydronephrosis. The UPJ obstruction can be due to various factors, including congenital anomalies, ureteral stones, or inflammation.

**Why Each Wrong Option is Incorrect**
* **Option A:** IVC thrombosis is more likely to cause bilateral hydronephrosis due to its central location and potential to affect both kidneys.
* **Option B:** Renal vein thrombosis typically affects the kidney's function but may not cause significant hydronephrosis unless there is a concurrent ureteral obstruction.
* **Option C:** Bladder outlet obstruction, such as benign prostatic hyperplasia, can cause bilateral hydronephrosis but is less likely to cause unilateral hydronephrosis.

**Clinical Pearl / High-Yield Fact**
The most common cause of unilateral hydronephrosis in adults is a UPJ obstruction, which is often asymptomatic in its early stages. A key clinical clue is the presence of flank pain and a palpable mass, which may indicate a more advanced obstruction.

**Correct Answer:** C.