Which cavity is located deep to the breastbone?

Mediastinum is the correct answer because it refers to the central compartment of the thoracic cavity, situated between the two lungs and posterior to the sternum (breastbone). This area houses vital structures, including the heart, trachea, esophagus, and major blood vessels. Its location deep to the breastbone is crucial, as it is surrounded by the thoracic cavity and serves as a conduit for many essential anatomical structures.

The pleural cavity is the space between the pleurae of the lungs, which allows for lung expansion and contraction during breathing but does not encompass the heart or major vessels. The pericardial cavity is specifically the space surrounding the heart and is located within the mediastinum but not deep to the breastbone itself. The abdominal cavity pertains to the space containing abdominal organs, located much lower than the structures in the mediastinum. Understanding these anatomical relationships clarifies why the mediastinum is the most appropriate choice for this question.