The narrowest pa of the ureter is at:
Uretero-pelvic junction
Iliac vessel crossing
Pelvic ureter
Ureterovesicle junction
The narrowest lumen is where the ureter enters the bladder wall (ureterovesicle junction) and may be responsible for arresting the passage of stones of as little as 2-3 mm. Note: Sometime the question may not have the option of ureterovesical junction, in that case the intramural ureter (the pa inside detrusor) may be taken as the answer.
GIT 1