Which vascular structure courses posterior to the superior mesenteric artery?

The left renal vein is the correct answer in this context because it anatomically courses posterior to the superior mesenteric artery (SMA). In the abdominal cavity, the SMA arises from the aorta at the level of the L1 vertebra and travels downward to supply blood to the intestines. As the left renal vein is positioned to the left of the aorta, it passes below the SMA before draining into the inferior vena cava. This relationship is critical to understand for identifying structures during sonographic examinations and avoiding potential complications during procedures.

In contrast, the splenic vein typically runs anterior to the SMA as it collects blood from the spleen and then drains into the portal vein, which is located behind the pancreas, further distancing it from a direct posterior relationship with the SMA. The splenic artery, which supplies blood to the spleen, also runs in a different course and does not have a posterior relationship to the SMA. The left gastric artery, while branching from the celiac trunk, is located more anteriorly in relation to the SMA, thus not fitting the criteria of being posterior to it. Understanding these anatomical relationships is fundamental for effective vascular assessment in sonography.