Internal pudendal
The rectal venous plexus is one of the four portal/systemic anastomoses. Blood from the portal system can flow into the venous system at this junction. This means that portal blood, from the superior rectal vein, could flow through the rectal venous plexus, into the inferior rectal vein and into the systemic venous drainage. Now, you just need to figure out what the inferior rectal vein drains into. And it drains into the internal pudendal vein, so that's the answer. See Netter plate 370 for a picture illustrating this concept of the portal/systemic anastomosis in the rectum.
The external iliac vein is one of the two branches of the common iliac vein (along with the internal iliac vein). However, the internal iliac vein and its tributaries (including the pudendal vein) are much more important in draining the pelvic structures. The inferior gluteal vein is a branch of the anterior division of the internal iliac vein--it drains gluteus maximus. The inferior mesenteric vein is part of the portal venous system--it gives rise to the superior rectal veins, but not the inferior rectal veins!