Final answer:
Underexposing a digital image receptor in radiography leads to increased quantum noise, a grainy appearance due to insufficient signal capture.
Step-by-step explanation:
Underexposing a digital image receptor in radiography can lead to increased quantum noise, which is the correct answer from the given options. Quantum noise, also known as image noise or photon noise, is the result of insufficient signal due to underexposure, where the individual X-ray photons are not captured in sufficient quantity to produce a clear image. This leads to a grainy or mottled appearance, reducing the diagnostic quality of the image. The Moire effect is related to grid misalignment and does not directly result from underexposure. Saturation occurs when too many photons hit the detector and it becomes unresponsive to further exposure. Fogging of the film suggests a background signal, often due to radiation, but is more a concern with traditional film rather than digital systems.