Which diagnostic finding may appear normal in a radiology exam?

A pulmonary embolus can sometimes be difficult to visualize on a standard radiology exam, resulting in a finding that appears normal even when a significant obstruction is present within the pulmonary circulation. This is due to the nature of the condition; frequently, the actual blockage may not produce any alterations in the lung tissue or pleural space that would be evident on a simple X-ray. Furthermore, standard chest X-rays may not show changes until a significant amount of lung tissue is affected or other complications arise.

In contrast, pneumonia, atelectasis, and pleural effusion typically present with identifiable changes on radiological exams. Pneumonia often shows infiltrates, atelectasis commonly results in volume loss of the affected lung segment which is usually visible, and pleural effusion can be seen as blunting of the costophrenic angles or as fluid levels within the pleura. Thus, the pulmonary embolus can indeed be a hidden diagnosis, making it a unique case among the options listed.