Which prefix indicates a direction toward something?

The prefix that indicates a direction toward something is "ad-." This prefix comes from Latin, meaning "to" or "toward." It is often used in medical terminology to denote movement or direction toward a structure or issue. For instance, in terms like "adduction," it refers to moving a body part closer to the midline of the body.

In contrast, "ab-" typically signifies movement away from something, as seen in "abduction," which means moving a body part away from the midline. The prefix "an-" can indicate absence or negation rather than direction. Lastly, "bi-" means two and does not indicate a directional movement or position. Thus, "ad-" is the correct choice for indicating direction toward something.