<s>[INST] context: The portal vein is compressed in its passage through the hepatoduodenal ligament, the anterior border of the omental (epiploic) foramen (of Winslow). The veins of Retzius are located along the sides of the abdominal walls and communicate between tributaries of retroperitoneal parts of the gastrointestinal tract and veins of the body wall. In portal hypertension the portal blood cannot pass freely through the liver, and the portal-caval tributaries and their anastomoses become dilated. The inferior epigastric veins anastomose with the paraumbilical veins, which is the first branch off the hepatic portal vein. These would be the first affected in portal hypertension. The inferior vena cava is the main route of blood return to the right atrium and is posterior to the omental (epiploic) foramen (of Winslow); it would not likely be compressed due to herniation through the foramen. Compression of the proper hepatic artery in the hepatoduodenal ligament would not result in dilation of veins of Retzius but could conceivably diminish blood supply to the gallbladder and liver. Common bile duct compression would result in jaundice and increased serum bilirubin. The cystic duct joins with the hepatic duct to form the common bile duct. Compression of this would lead to an inflamed gallbladder (cholecystitis). [/INST] question: A 41-year-old man entered the emergency department with abdominal trauma after his motorcycle was struck by a hit-and-run automobile driver. One week following emergent surgery the patient was released from the hospital. Two months postoperatively the patient complained of abdominal pain. A CT scan examination demonstrated an internal hernia in which the hepatic flexure of the colon had herniated through the epiploic (omental) foramen (of Winslow). Gastrointestinal veins appeared to be markedly dilated, including the veins forming anastomoses between the portal and caval systems (veins of Retzius). Which of the following structures is most likely compressed? | options: A: Portal vein B: Inferior vena cava C: Hepatic artery D: Common bile duct | answer: A </s>