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?

Explanation

## Why "Anterior nutcracker compression of the left renal vein between the aorta and SMA, causing venous hypertension and hematuria" is right The left renal vein is anatomically longer than the right and crosses anterior to the aorta in the acute angle between the aorta and the superior mesenteric artery (SMA). This anatomical configuration places the left renal vein at risk for anterior nutcracker compression, where the vessel is squeezed between these two arterial structures. The resulting venous hypertension leads to the classic triad of left flank pain, hematuria, and varicocele in males. The elevated flow velocity on Doppler imaging confirms hemodynamic compromise at this site. This is a well-recognized anatomical variant that can cause symptomatic left renal vein compression (Sutton Radiology 7e Ch 32; Bailey & Love 28e Ch 75). ## Why each distractor is wrong - **Posterior nutcracker compression of the left renal vein behind the aorta, leading to renal infarction**: Posterior nutcracker compression is rare and typically involves compression behind the aorta by the aorta and vertebral column. The anterior position of the left renal vein crossing anterior to the aorta (as marked in the diagram) makes anterior compression the relevant pathology. Renal infarction would present with acute severe flank pain and elevated LDH, not the chronic hematuria described here. - **Compression of the left renal artery by the aorta, resulting in renovascular hypertension**: The clinical presentation (hematuria, flank pain) and imaging findings (elevated flow velocity in the vein, not the artery) point to venous, not arterial, compression. Renal artery compression would cause renovascular hypertension as the primary feature, not hematuria. The question specifically marks the renal vein (B), not the artery. - **Entrapment of the left ureter in the angle between the aorta and IVC**: The ureter is a posterior retroperitoneal structure that does not cross in the angle between the aorta and SMA. Ureteric entrapment would cause obstructive uropathy with hydronephrosis and elevated creatinine, not the hemodynamic hematuria seen here. The marked structure (B) is the renal vein, not the ureter. **High-Yield:** Left renal vein crosses anterior to aorta between aorta and SMA → anterior nutcracker compression → left flank pain + hematuria + varicocele (male). [cite: Sutton Radiology 7e Ch 32; Bailey & Love 28e Ch 75]