Pg 366 FA 2020.
Hemorrhoids are dilatations of arteriorvenous plexuses in the rectum. Blockages on the arterial side will not cause the plexus to fill while venous thromboses will cause a back up of fluid.
Clues for hinting which vein are the location at the anal margin and that he was in extreme pain. External hem's will be painful, pointing towards inferior rectal as the answer since it drains below the pectinate line where external hem's are found.