A 60-yeаr-оld mаle presents with chrоnic bаck pain, bilateral leg edema, and unexplained weight lоss. Sonographic evaluation reveals a dense, ill-defined, hypoechoic midline mass enveloping the abdominal aorta and inferior vena cava (IVC) near the aortic bifurcation. The anterior wall of the aorta appears tethered, and there is associated bilateral hydronephrosis due to ureteral compression. This appearance is most characteristic of?