A 45-year-old female is admitted to the hospital with symptoms of an upper bowel obstruction. Upon CT examination it is found that the third (transverse) portion of the duodenum is being compressed by a large vessel. Which of the following vessels will most likely be causing the compression?

The superior mesenteric artery arises from the aorta, behind the neck of the pancreas, and descends across the uncinate process of the pancreas and the third part of the duodenum before it enters the root of the mesentery behind the transverse colon. It can compress the third part of the duodenum. The inferior mesenteric artery passes to the left behind the horizontal portion of the duodenum. The inferior mesenteric vein is formed by the union of the superior rectal and sigmoid veins and it does not cross the third part of the duodenum. The portal vein is formed by the union of the splenic vein and the superior mesenteric vein posterior to the neck of the pancreas. It ascends behind the bile duct and the hepatic artery within the free margin of the hepatoduodenal ligament. The splenic vein is formed by the tributaries from the spleen and is superior to the third part of the duodenum.