A 28-year-old woman presents with left flank pain and microscopic hematuria. Axial CT at the level of the renal hila (L1-L2) shows the left renal vein marked as **B** crossing anterior to the aorta in the angle between the aorta and superior mesenteric artery. Doppler ultrasound confirms elevated flow velocity in this vessel. Which of the following best explains the anatomical basis for her clinical presentation?