What combining form means 'within'?

• A. Infra-
• B. Endo-
• C. Amyl/o
• D. Lact/o

Answer: (B)

Endo- is the combining form that means within or inside. It comes from Greek, and it’s used to indicate something located inside a structure. For example, endoscope is an instrument used to look inside the body, endocardium is the inner lining of the heart, and endometrium is the inner lining of the uterus. The other forms have different meanings—infra- means below or beneath, amyl/o refers to starch, and lact/o refers to milk—so they don’t convey the idea of “within.”