**Core Concept**
Pyonephrosis refers to a condition where a hydronephrotic kidney becomes infected, leading to the accumulation of pus and bacteria within the renal pelvis and calyces. This condition often results from an obstructed urinary tract, allowing bacteria to ascend and colonize the kidney.

**Why the Correct Answer is Right**
Pyonephrosis typically occurs in the setting of an obstructed urinary tract, such as in cases of ureteral stones, tumors, or strictures. The obstruction prevents the normal flow of urine, allowing bacteria to multiply and cause infection. As a result, the renal pelvis and calyces become dilated, leading to the accumulation of pus and bacteria. This condition can be life-threatening if left untreated, and prompt surgical intervention is often necessary to restore urinary drainage and prevent further complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Pyonephrosis can be caused by an obstructed urinary tract, so this statement is true.
**Option B:** Pyonephrosis often results from an obstructed urinary tract, which allows bacteria to ascend and colonize the kidney, making this statement true.
**Option C:** This option is incorrect because pyonephrosis typically occurs in the setting of an obstructed urinary tract, not in the absence of obstruction.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that pyonephrosis is a medical emergency that requires prompt surgical intervention to restore urinary drainage and prevent further complications. Delayed treatment can lead to severe consequences, including sepsis and renal failure.

**Correct Answer: C. This option is incorrect because pyonephrosis typically occurs in the setting of an obstructed urinary tract, not in the absence of obstruction.**