**Core Concept**
Idiopathic retroperitoneal fibrosis (IRF) is a rare inflammatory condition characterized by the proliferation of fibrous tissue in the retroperitoneum, leading to encasement and potential obstruction of adjacent structures, including the ureters, aorta, and major blood vessels.

**Why the Correct Answer is Right**
The most common clinical presentation of IRF is ureteral obstruction due to the encasement of the ureters by the fibrotic tissue. This can lead to symptoms such as flank pain, hematuria, and obstructive uropathy. The obstruction can be either unilateral or bilateral, resulting in acute kidney injury if not promptly addressed. The fibrotic tissue in IRF is thought to be driven by an autoimmune response, with some cases associated with the presence of an IgG4-related disease.

**Why Each Wrong Option is Incorrect**
**Option A:** While abdominal mass is a possible presentation of IRF, it is not the most common clinical presentation.

**Option B:** Flank pain is a common symptom of IRF, but it is not the most common clinical presentation.

**Option C:** Hematuria can be a symptom of IRF due to ureteral obstruction, but it is not the most common clinical presentation.

**Clinical Pearl / High-Yield Fact**
One important clinical point to remember is that IRF can mimic other conditions such as lymphoma, metastatic disease, and aortic aneurysm, making a high index of suspicion and prompt imaging studies essential for diagnosis.

**Correct Answer:** A. Abdominal mass.