The most common causes differ by age: Children: Anatomic abnormalities (including posterior urethral valves or stricture and stenosis at the ureterovesical or ureteropelvic junction) Young adults: Calculi Older adults: Benign prostatic hyperplasia (BPH) or prostate cancer, retroperitoneal or pelvic tumors (including metastatic cancer), and calculi Obstruction may occur at any level, from the renal tubules (casts, crystals) to the external urethral meatus.