Ureteropelvic junction (UPJ) obstruction is when part of the kidney is blocked. Most often it is blocked at the renal pelvis. This is where the kidney attaches to one of the ureters (the tubes that carry urine to the bladder). The blockage slows or stops the flow of urine out of the kidney. Urine can then build up and damage the kidney.

I think specifically they are referring to the ureteropelvic junction, which is the LAST to canalize and can lead to congenital obstruction. Also, it is the MC cause of prenatal hydronephrosis. This is detected by ultrasound.

Also important, the 3 MC points of ureteral obstruction: ureteropelvic junction, pelvic inlet, and ureterovesical junction.

FA 2020: 578 and 581.