Which prefix refers to "backward" movement in medical terminology?

The prefix that refers to "backward" movement in medical terminology is "retro-." This prefix is commonly used in various medical terms to indicate a return to a previous state or a movement towards the back. For instance, "retrograde" refers to situations where a process or function moves backward, such as the backward flow of blood in certain medical conditions.

Understanding the context and usage of "retro-" in terms such as "retroperitoneal" (located behind the peritoneum) reinforces its meaning related to backward or behind. The other prefixes listed, while important in medical terminology, do not convey the concept of backward movement. "Pat-" generally pertains to a condition relating to a flat or level surface, "pneumo-" relates to air or lungs, and "photo-" pertains to light. Thus, "retro-" clearly stands out as the correct choice for indicating backward movement.