**Core Concept**
The posterior mediastinum is a region of the mediastinum that contains the esophagus, thoracic aorta, thoracic duct, and sympathetic chain. A posterior mediastinal opacity on chest X-ray may indicate the presence of an abnormality in one of these structures.

**Why the Correct Answer is Right**
The correct answer is likely related to a condition affecting the esophagus or thoracic aorta. The esophagus can be involved in conditions such as esophageal cancer, esophageal diverticula, or esophageal duplication cysts. The thoracic aorta can be involved in conditions such as aortic dissection, aortic aneurysm, or aortic thrombosis. These conditions can cause opacification of the posterior mediastinum on chest X-ray due to the accumulation of fluid, blood, or tumor cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely incorrect because it does not specify a condition that is typically associated with a posterior mediastinal opacity. While lung nodules can cause opacities on chest X-ray, they are not typically located in the posterior mediastinum.

**Option B:** This option is likely incorrect because it is too nonspecific. Many conditions can cause pleural effusions, including pneumonia, heart failure, and cancer. However, a pleural effusion would typically appear as a fluid level in the costophrenic angle on lateral view, rather than a posterior mediastinal opacity.

**Option C:** This option is likely incorrect because it does not specify a condition that is typically associated with a posterior mediastinal opacity. While mediastinal lymphadenopathy can cause opacities on chest X-ray, it is typically more anterior in location.

**Option D:** This option is likely correct because it specifies a condition that is typically associated with a posterior mediastinal opacity. Aortic dissection can cause opacification of the posterior mediastinum on chest X-ray due to the accumulation of blood in the mediastinum.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the location and characteristics of a mediastinal opacity on chest X-ray can provide clues about the underlying diagnosis. A posterior mediastinal opacity is more likely to be related to a condition affecting the esophagus or thoracic aorta.

**Correct Answer: D. Aortic dissection.**