What directional term would indicate a sunburn on a patient's back?

The term that indicates a sunburn on a patient's back is "posterior." In anatomical terminology, "posterior" refers to the back side of the body. Therefore, if a patient has a sunburn on their back, it is accurately described as being on the posterior surface. This directional term helps to specify locations of structures in relation to one another on the body's three-dimensional axis.

Other options, such as "anterior," "lateral," and "superior," do not correctly describe the location of the sunburn on the back. "Anterior" would refer to the front aspect of the body, "lateral" indicates a position further away from the midline of the body, and "superior" is used to describe a location that is above another part. Therefore, "posterior" is the most appropriate term to describe the area affected by the sunburn on the back.