If a radiograph were made of an average-size knee using automatic exposure control (AEC) and all three photocells were selected, the resulting radiograph would demonstrate?

When using automatic exposure control (AEC), the system is designed to terminate the exposure once it has received the necessary amount of radiation to produce a diagnostic quality image. If all three photocells are selected while imaging an average-size knee, the AEC will average the signals from these cells, potentially leading to unintended exposure results because it might detect more surrounding tissues than just the knee itself.

This situation can occur particularly if the knee is positioned such that the selected photocells receive significant contributions from adjacent areas, such as the surrounding soft tissue or bone. If those areas absorb more radiation and signal the AEC, the exposure might terminate too early, leading to an underexposed image of the knee itself.

In this context, having multiple photocells active can lead to miscalculations in what the AEC 'sees' as the necessary exposure, thus resulting in inadequate image brightness and detail specifically in the region of interest, which in this case is the knee. Therefore, the outcome is likely to be an underexposed image where key anatomical detail may be obscured or not visible enough for accurate diagnosis.