The Collaborative International Dictionary
Recto-
Recto- \Rec"to-\ (r?k"t?*). A combining form indicating connection with, or relation to, the rectum; as, recto-vesical.
Douglas Harper's Etymology Dictionary
recto-
word-forming element meaning "pertaining to or involving the rectum," before vowels rect-, from comb. form of rectum.
Wiktionary
recto-
pre. (context anatomy English) rectum