Which of the following factors is most likely to result in a longer exposure time when using AEC?

Choosing decreased milliamperage (mA) as the factor most likely to result in a longer exposure time when using Automatic Exposure Control (AEC) is accurate because mA directly influences the amount of x-ray photons produced during the exposure. AEC systems are designed to terminate the exposure when a specific amount of radiation has been detected. If the mA is decreased, the x-ray tube produces fewer photons per unit of time, which means that it will take a longer time to reach the pre-set exposure level. This leads to a lengthened exposure time.

Conversely, options such as higher kilovoltage (kVp) would tend to reduce exposure time because increasing kVp enhances the efficiency of photon generation and may allow the AEC to reach the exposure threshold more quickly. A lower grid ratio usually leads to less scatter radiation being absorbed, which can allow for quicker termination of the exposure when sufficient primary beam exposure is achieved. Increased source-to-image distance (SID) can also extend exposure time due to the inverse square law, where the intensity of radiation decreases as the distance from the source increases. However, the most direct and significant impact on increasing exposure time in this context is related to decreased milliamperage.