Perineum: a free MRCS Part A question

Internal haemorrhoids are enlargements of the anal cushions, vascular plexuses in the submucosa above the dentate line at the left lateral, right anterior and right posterior positions (3, 7 and 11 o'clock in the lithotomy position). They are fed by terminal branches of the superior rectal artery and drain through the superior rectal vein to the inferior mesenteric vein and so to the portal system, which is why the anal canal is a site of portosystemic anastomosis. The middle rectal veins drain the muscular wall of the lower rectum to the internal iliac veins, and the inferior rectal veins drain the canal below the dentate line to the internal pudendal veins, both systemic. The median sacral vein drains the back of the rectum and sacrum. Reference: Last's Anatomy 12e.