What is the most likely explanation if a series of chest radiographs appear darker towards one side?

The most likely explanation for a series of chest radiographs appearing darker towards one side is that the X-ray beam is usually more intense on one side of the field. In radiographic imaging, variations in exposure can occur due to factors such as beam collimation, patient positioning, and the geometry of the X-ray equipment. If the X-ray beam is not evenly distributed, it may lead to areas of overexposure on one side of the chest radiograph, resulting in darker images in those regions.

Factors such as the angle of the X-ray tube, the distance between the tube and the patient, and the alignment of the X-ray film or digital detector can also contribute to unequal intensity. Such discrepancies in exposure can lead to diagnostic challenges, as the visual interpretation of structures may be affected by the varying levels of radiographic density.

The other options, such as low fixative or developer in the processor, may affect the quality of the developed films but would not typically lead to a consistent pattern of darkness on one side across multiple images. Similarly, an improperly closing cassette would likely cause more random artifacts rather than a systematic appearance of darkness favoring one side.