Which two locations are the most common sites for obstruction in the ureter?

The two most common sites for obstruction in the ureter are indeed at the ureteropelvic junction and the ureterovesical junction.

The ureteropelvic junction is the area where the renal pelvis transitions into the ureter. This location is particularly prone to obstruction due to anatomical variations, such as the presence of a crossing blood vessel or simply due to the natural angle and configuration of these structures.

The ureterovesical junction is where the ureter connects to the bladder. This area can experience obstruction from various causes, including bladder outlet obstruction or the presence of stones that may obstruct the passage as the urine drains into the bladder.

Together, these two junctions represent the most common sites for occlusion of the ureteral pathway, making this answer accurate and clinically relevant. Recognizing these sites is crucial for imaging studies and when considering the differential diagnoses for renal colic or other symptoms related to urinary obstruction.