While processing a set of BW radiographs, three are of good quality but one is completely blank. What is the most likely cause?

The scenario describes a blank radiograph that indicates that the film did not receive any exposure to x-rays during the imaging process. This absence of exposure is the most probable reason for the completely blank film. In dental radiography, if the film is not exposed properly to radiation, it will not record any of the details required and will appear entirely clear upon processing.

In contrast, incorrect processing temperature, underdevelopment, or using an incorrect film type would typically lead to films that have some level of image, whether poorly exposed, overly light, or of inadequate contrast. These issues often manifest as faint images or images with poor detail, rather than complete lack of an image. Therefore, the definitive characteristic of a blank film strongly points to an issue with x-ray exposure, making it the most logical and accurate answer.