Final answer:
An infiltrate that outlines the horizontal fissure of the lung is located between the right superior and middle lobes, indicating a pathological process such as pneumonia in these areas. The horizontal fissure is specific to the right lung and its visualization on a radiograph is indicative of disease.
Step-by-step explanation:
If an infiltrate in the lung outlines the horizontal fissure, it would be located between the lobes of the lung that are separated by this fissure. Specifically, the horizontal fissure is found in the right lung and separates the right superior lobe from the right middle lobe. Thus, an infiltrate outlining this fissure indicates a pathological process, such as pneumonia, involving these two lobes.
Since each lung is composed of smaller units called lobes, and the fissures separate these lobes from each other, the outline of the horizontal fissure by an infiltrate could signal the presence of fluid-filled pockets within these regions. These pockets may have been shown as a white area on a chest radiograph, which typically would be black, indicating the presence of such a pathological process.