Final answer:
On a decubitus x-ray, the area of interest is the affected side down. The patient lies on their side, with the side of interest closest to the surface.
Step-by-step explanation:
The area of interest on a decubitus x-ray is on the affected side down. Decubitus means lying down, and there are different positions in which a person can lie down. In the context of a decubitus x-ray, the area of interest would be the side of the body that is facing up, as this is the position being examined. In a decubitus position, a patient is lying on his or her side, so when the x-ray is taken, the side of interest is that which is closest to the table or resting surface. This position helps in visualizing certain structures by taking advantage of gravity, allowing free floating substances, such as air or fluid, to settle and become more clearly defined.