A 52-year-old man from Delhi presents to the emergency department with acute left-sided flank pain radiating to the groin, accompanied by haematuria. Non-contrast CT abdomen shows a 8 mm stone in the left ureter at the level of the pelvic brim. The radiologist notes that the stone is lodged at the most common site of ureteric constriction. During surgical planning, the urologist reviews the anatomical relations of the ureter at this level. Which of the following structures crosses anterior to the ureter at the pelvic brim, potentially contributing to ureteric obstruction at this site?