Middle rectal vein-there are four portal-caval anastomoses in the body. first, between the superior rectal veins in the portal system and the middle and inferior rectal veins in the caval system. second, between the esophageal veins that go to the left gastric vein (portal) and the esophageal veins that go to the azygos system (caval). third, between the paraumbilical veins of the portal system and the veins of the anterior abdominal wall that drain into the inferior vena cava. fourth, between the colic veins of the portal system and the retroperitoneal veins of the caval system. so middle rectal is the right answer. the superior gluteal vein, renal vein, and inferior phrenic vein are all part of the caval system; the splenic vein is part of the portal system.
