The mediastinum is best described as which of the following?

• A. The mass of tissues and organs lying behind the sternum, between the lungs
• B. The membrane surrounding the lungs
• C. The space within the pericardial sac
• D. The outermost layer of the heart wall

Answer: (A)

The mediastinum is the central chest compartment between the lungs that runs from the sternum to the vertebral column and from the thoracic inlet to the diaphragm. It contains the heart, great vessels, trachea, esophagus, thymus, and other structures. Describing it as the mass of tissues and organs lying between the lungs (and behind the sternum) best captures its location and contents.

The other descriptions refer to different structures: the membrane around the lungs is the pleura; the space inside the pericardial sac is the pericardial cavity; and the outermost layer of the heart wall is the epicardium.