## Core Concept
Ormond's disease, also known as **idiopathic retroperitoneal fibrosis**, is a rare condition characterized by the proliferation of fibrous tissue in the retroperitoneum, leading to encasement and potential obstruction of structures within the retroperitoneum, such as the ureters and major blood vessels.

## Why the Correct Answer is Right
The correct answer, **idiopathic retroperitoneal fibrosis**, is the definition of Ormond's disease. This condition is marked by the abnormal growth of fibrous tissue in the retroperitoneal space, which can cause a variety of symptoms due to the involvement of nearby structures. The exact cause is often unknown (idiopathic), but it can be associated with certain drugs, autoimmune conditions, and malignancies.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not correspond to the definition of Ormond's disease.
- **Option B:** This option is incorrect as it also does not match the definition of Ormond's disease.
- **Option C:** Although not provided, any other option would be incorrect if it does not accurately describe idiopathic retroperitoneal fibrosis.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that Ormond's disease often presents with **flank pain** and can lead to **hydronephrosis** due to ureteral obstruction. Early diagnosis and treatment are crucial to prevent complications such as kidney failure.

## Correct Answer Line
**Correct Answer: D. idiopathic retroperitoneal fibrosis**