**Core Concept**
Xanthogranulomatous pyelonephritis (XGP) is a rare form of chronic pyelonephritis characterized by the destruction of renal parenchyma and the formation of xanthomatous granulomas due to an intense inflammatory response. This condition is often associated with long-standing urinary tract obstruction and infection.

**Why the Correct Answer is Right**
XGP is typically seen in the context of chronic urinary tract obstruction, which leads to renal pelvis and calyceal dilation, and subsequent bacterial infection. The intense inflammatory response results in the destruction of renal parenchyma and the formation of xanthomatous granulomas. These granulomas are composed of lipid-laden macrophages, which give the characteristic yellowish appearance to the affected tissue.

**Why Each Wrong Option is Incorrect**
* **Option A:** XGP is indeed often associated with long-standing urinary tract obstruction, which is a key factor in the development of this condition.
* **Option B:** XGP is typically a unilateral condition, but it can be bilateral in rare cases. However, the statement is not false, so this option might not be the correct answer.
* **Option C:** XGP is indeed a rare form of chronic pyelonephritis, so this statement is true.

**Clinical Pearl / High-Yield Fact**
XGP can mimic other renal conditions, such as renal cell carcinoma or abscesses, on imaging studies, so a high index of suspicion is necessary for diagnosis.

**Correct Answer: B.**