rectum (n.), rectal (adj.)

https://doi.org/10.1351/goldbook.11168
Terminal part of the large intestine, ending at the anus.
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 799 (https://doi.org/10.1515/pac-2015-1202)