Which prefix means 'Out; on the outside'?

• A. Ecto-
• B. Exo-
• C. Extra-
• D. Ultra-

Answer: (A)

Ecto- denotes outside or external position, coming from the Greek ectos meaning outside. It’s the form you see in terms like ectoderm (the outermost embryonic layer), ectoplasm (the outer part of cytoplasm), and ectopic (located outside the normal position). This use makes it thebest fit for indicating something on the exterior or outward relative to the body. While exo- also means outside and appears in words like exocrine or exoskeleton, ecto- is the more specific combining form used to describe outer or external relations in many medical terms. Extra- and ultra- don’t carry the same precise sense of being on the outside; extra- means outside of or beyond in a broader sense, and ultra- means beyond in degree.