What is the correct combining form for rectum or anus?

The combining form for rectum or anus is "proct/o." This term derives from the Greek word "proktos," which means "anus" or "rectum." In medical terminology, "proct/o" is frequently used in terms associated with conditions, procedures, and anatomy related to the lower gastrointestinal tract, such as proctology, which is the study and treatment of disorders of the rectum and anus.

The other choices do not relate to the rectum or anus. "Psych/o" pertains to the mind or mental processes, "rhin/o" refers to the nose, and "vas/o" relates to vessels, especially blood vessels. Each of these combining forms serves a distinct purpose in medical terminology, associating with entirely different anatomical or physiological aspects. Thus, "proct/o" is the correct form to denote matters concerning the rectum or anus.