The prefix meaning "within" is which?

• A. In-
• B. Inter-
• C. Intra-
• D. Infra-

Answer: (D)

The concept tested is how prefixes convey spatial relationships. Within a structure or space is most precisely expressed by the prefix intra-, as in intracellular, intracardiac, or intraoral. It signals being inside or inside the boundaries of something.

Infra-, by contrast, means below or beneath (infrared, infrastructure), not inside. The general prefix in- can mean into or inside in many terms, but when you want to indicate being inside a system or area, intra- is the correct choice.