Which of the following structures is NOT contained in the mediastinum?

The mediastinum is the central compartment of the thoracic cavity, located between the lungs. It contains vital structures such as the heart, esophagus, trachea, mainstem bronchi, and various blood vessels and nerves.

The diaphragm, while crucial for respiration, is actually a muscular structure that separates the thoracic cavity from the abdominal cavity. It does not lie within the mediastinum itself but instead forms the floor of the thoracic cavity. Thus, it is accurate to state that the diaphragm is not contained within the mediastinum, making it the correct answer.

Understanding the anatomy of the mediastinum provides insight into its role in housing essential thoracic structures and how they interact spatially within the body. Knowing the distinction between structures located in the mediastinum and those that are adjacent or supportive, such as the diaphragm, helps clarify the functional organization of the thoracic cavity.