Final answer:
Exposure factors for the shortest-scale contrast include higher mAs, a moderate kV, a fast film-screen system, a high grid ratio, and a smaller field size, with the fourth given combination satisfying these conditions.
Step-by-step explanation:
The combination of exposure factors that most likely will contribute to producing the shortest-scale contrast in radiographic imaging includes high milliampere-seconds (mAs), higher kilovoltage peak (kV), a film-screen system with high relative speed, and a high grid ratio along with a smaller field size. In the given options, combination 4, which consists of mAs: 20; kV: 80; Film-screen system: 400; Grid ratio: 10:1; Field size: 8 × 10 in, represents the parameters that would lead to the shortest scale of contrast due to the higher mAs value which increases patient exposure, the moderate kV that is not too high to produce too much penetration, a fast film-screen system speed of 400 that requires less exposure, and a high grid ratio of 10:1 effective in absorbing scattered radiation. Additionally, a smaller field size minimizes the volume of tissue irradiated, further contributing to reduced scatter and hence shorter-scale contrast.