Which artery is most likely causing hydronephrosis due to external compression at the pelvic brim in a 76-year-old woman?

The common iliac artery is the most likely vessel causing hydronephrosis due to external compression at the pelvic brim. Hydronephrosis, which refers to the swelling of a kidney due to a build-up of urine, can occur when there is an obstruction to urine flow. In this case, the pelvic brim is the area where the common iliac arteries run as they bifurcate from the abdominal aorta, and they can exert pressure on the ureters due to their anatomical positioning.

In older patients, the risk of hydronephrosis can increase due to age-related factors such as the presence of abdominal masses, pelvic tumors, or vascular changes that may lead to compression of adjacent structures. The female anatomy often includes a broader pelvicalyceal system, and in this case, as the ureters cross the pelvic brim in close proximity to the common iliac arteries, any external compression from these arteries can lead to obstructed urine flow and hydronephrosis.

Furthermore, while the abdominal aorta is a major vessel supplying blood to the lower body, it is typically too far posterior to directly cause compression at the pelvic brim. The renal arteries are located more superiorly and carry blood to the kidneys themselves, rather than causing compression