**Core Concept**
The posterior mediastinum is a region of the mediastinum that contains the esophagus, the descending thoracic aorta, the thoracic duct, and the sympathetic chain. Posterior mediastinal opacities on chest radiographs can result from various pathologies affecting these structures.

**Why the Correct Answer is Right**
The correct answer, **D. Neuroblastoma**, is a type of extracranial solid tumor that often originates from the adrenal glands but can also arise from the sympathetic chain in the posterior mediastinum. Neuroblastomas can cause posterior mediastinal opacities due to the tumor's mass effect, which may lead to an increase in opacity on chest radiographs.

**Why Each Wrong Option is Incorrect**
* **Option A:** Esophageal diverticulum is a false diverticulum that protrudes from the wall of the esophagus, but it is typically located in the posterior mediastinum and would not cause an opacity on PA and lateral views of the chest radiograph.
* **Option B:** Ascending aortic aneurysm is a dilation of the ascending aorta, which is located in the anterior mediastinum, and would not cause an opacity in the posterior mediastinum.
* **Option C:** Pericardial effusion is an accumulation of fluid in the pericardial space, which surrounds the heart and is located in the anterior mediastinum, and would not cause an opacity in the posterior mediastinum.

**Clinical Pearl / High-Yield Fact**
Neuroblastomas are often associated with systemic symptoms such as weight loss, fatigue, and anemia, and may also cause bone marrow metastases, which can lead to pancytopenia.

**Correct Answer:** D. Neuroblastoma