**Core Concept**
The posterior mediastinum is a compartment of the thoracic cavity that contains various structures including the esophagus, thoracic aorta, and the sympathetic chain. The pericardial sac, a fibroserous sac, encloses the heart and its roots. Understanding the anatomical relationship between these structures is crucial for various medical procedures and conditions.

**Why the Correct Answer is Right**
The correct answer is the thoracic duct, which is a crucial structure in the posterior mediastinum. The thoracic duct ascends through the posterior mediastinum, closely applied to the posterior surface of the pericardial sac. This anatomical relationship is significant because the thoracic duct is at risk during cardiac surgeries, and its injury can lead to devastating consequences. The thoracic duct is a key structure in the lymphatic system, responsible for draining lymph from the lower limbs, abdomen, and pelvis into the bloodstream.

**Why Each Wrong Option is Incorrect**
* **Option A:** The esophagus is a muscular tube that carries food from the pharynx to the stomach, but it is not closely applied to the posterior surface of the pericardial sac. The esophagus is actually located more anteriorly in the posterior mediastinum.
* **Option B:** The thoracic aorta is a major artery that arises from the left ventricle of the heart and descends through the posterior mediastinum. However, it is not closely applied to the posterior surface of the pericardial sac, but rather lies within the aortic hiatus of the diaphragm.
* **Option C:** The sympathetic chain is a ganglionic structure that lies along the thoracic and abdominal aorta, but it is not closely applied to the posterior surface of the pericardial sac. The sympathetic chain is more laterally located in the posterior mediastinum.

**Clinical Pearl / High-Yield Fact**
The thoracic duct is at risk during cardiac surgeries, particularly during procedures involving the posterior mediastinum, such as thoracotomy or aortic surgery. Injuries to the thoracic duct can lead to chylothorax, a condition characterized by the accumulation of lymphatic fluid in the pleural space.

**Correct Answer:** D. The thoracic duct.