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?