A 52-year-old man undergoes a contrast-enhanced CT abdomen for evaluation of chronic abdominal pain. On axial imaging at the level of the renal hilum, a well-defined, comma-shaped structure is identified anterior to the descending aorta and posterior to the superior mesenteric artery. The structure demonstrates homogeneous enhancement in the arterial phase. Which of the following anatomical structures is most likely being visualized?

Explanation

## Identification of the Left Renal Vein ### Anatomical Relationships The left renal vein is a key retroperitoneal structure with characteristic anatomical relationships on cross-sectional imaging: **Key Point:** The left renal vein courses anterior to the left renal artery and posterior to the superior mesenteric artery (SMA), creating a distinctive comma-shaped or curved appearance on axial CT. ### Imaging Characteristics | Feature | Left Renal Vein | Left Renal Artery | |---------|-----------------|-------------------| | **Position** | Anterior to artery | Posterior to vein | | **Course** | Anterior to aorta | Posterior to aorta | | **Enhancement pattern** | Venous phase (delayed) | Arterial phase (early) | | **Relationship to SMA** | Posterior | Anterior | | **Shape** | Comma-shaped | Straighter course | ### Why the Correct Answer is Correct The stem explicitly states "homogeneous enhancement in the arterial phase," which is a critical clue. However, the anatomical descriptor—comma-shaped, anterior to the aorta, posterior to the SMA—is pathognomonic for the **left renal vein**. The left renal vein is the only major retroperitoneal vessel at the renal hilum level that: 1. Has a comma or curved shape on axial imaging 2. Lies anterior to the descending aorta 3. Courses posterior to the SMA 4. Is located at the level of the renal hilum **Clinical Pearl:** The left renal vein is longer than the right (typically 5–7 cm) because it must cross the midline to reach the IVC. This longer course and the need to navigate around the SMA give it its characteristic curved appearance. ### High-Yield Anatomy Point **High-Yield:** On axial CT at the renal hilum level, remember the anterior-to-posterior sequence from left to right: **SMA → Left renal vein → Left renal artery → Aorta**. This mnemonic order helps distinguish the left renal vein (anterior structure) from the left renal artery (posterior structure). ### Venous vs. Arterial Enhancement Clarification **Warning:** The stem mentions "arterial phase enhancement," which might seem to favor an artery. However, in clinical practice, the left renal vein does enhance in the arterial phase due to rapid venous return from the kidney. The comma-shaped morphology and anterior position are the definitive anatomical clues that override the enhancement timing in this case.