If three BW films appear normal but one is clear, what is the probable cause?

In this scenario, when three bitewing (BW) films appear normal but one is clear, the most likely cause is that the button was not fully pressed during the exposure of that particular film. When the exposure button is not fully depressed, the film does not receive the necessary x-ray exposure, resulting in a clear or unexposed film. This is a common occurrence in radiographic procedures, where proper technique in operating the x-ray equipment is crucial for obtaining diagnostic images.

The other options, while plausible under different circumstances, do not directly address the specific situation of one film being clear among several that are normal. A malfunctioning x-ray machine would likely affect all films taken in that session rather than just one. Exposure of the film to light would typically lead to fogged images or overall darkening, rather than a clear film. Finally, if the processing solution were exhausted, it would likely cause problems for all films processed rather than just one, leading to inadequate development rather than complete lack of exposure. Thus, the explanation for why the button not being pushed completely is the most fitting for this situation revolves around the mechanics of capturing the x-ray image and the operator's technique.