**Core Concept**
Percutaneous nephrostomy is a minimally invasive procedure used to divert the flow of urine directly from the kidneys when normal flow is blocked. This technique is crucial in managing various urinary tract obstructions.

**Why the Correct Answer is Right**
The correct answer involves understanding the primary indications for percutaneous nephrostomy, which include relieving obstruction and facilitating diagnostic or therapeutic interventions. **Ureteral obstruction** is a key indication, as it prevents normal urine flow, potentially leading to complications like pyelonephritis or renal failure.

**Why Each Wrong Option is Incorrect**
**Option A:** While percutaneous nephrostomy can be used to access the kidney for stone removal, it's not the primary indication.
**Option C:** Renal tumor resection is a more invasive procedure that typically requires open surgery or laparoscopy.
**Option D:** Ischemic renal failure is managed with supportive care and addressing the underlying cause, not typically with percutaneous nephrostomy.

**Clinical Pearl / High-Yield Fact**
Percutaneous nephrostomy is a lifesaving procedure in cases of obstructive uropathy, allowing for the diversion of urine and prevention of further renal damage.

**Correct Answer:** B. Ureteral obstruction