What is the prefix that means 'beside'?

The prefix that means 'beside' is indeed the one that indicates a position in relation to something else. In medical terminology, "para-" is derived from Greek, where it often implies 'alongside' or 'beside.' This prefix is frequently used in various medical terms, such as "paralysis," which refers to a loss of movement alongside a structure.

The other prefixes refer to different concepts: "-ostomy" is related to creating an opening; "-dynia" refers to pain; and "-phobia" indicates an irrational fear. Each of these prefixes serves a distinct purpose in the medical lexicon, but none conveys the meaning of 'beside' like "para-." Therefore, "para-" is the correct choice in this context.