Final answer:
The combining form that means 'rectum and anus' is 'procto-,' derived from the Greek word for the anus.
Step-by-step explanation:
The combining form that refers to the rectum and anus is procto-. This prefix is derived from the Greek word proktos, which means anus or hindparts. In medical terminology, procto- is commonly used to form compound words that are associated with the rectum and anus. Understanding the anatomy involved, the rectum is the final straight portion of the large intestine that leads to the anus.