**Core Concept**
In a chest X-ray, homogenous opacity on one side with mediastinal shift to the opposite side suggests a significant accumulation of fluid in the pleural space, which is characteristic of pleural effusion. This condition can lead to increased density on the affected side and subsequent displacement of the mediastinum.

**Why the Correct Answer is Right**
Pleural effusion occurs when there is an abnormal collection of fluid in the pleural space, which can be due to various causes such as heart failure, malignancy, or infections. The fluid accumulation leads to increased opacity on the affected side, and the weight of the fluid causes the mediastinum to shift towards the opposite side. In this case, the homogenous opacity on the right side with mediastinal shift to the left suggests a significant pleural effusion.

**Why Each Wrong Option is Incorrect**
**Option A:** Collapse, also known as atelectasis, typically presents with a more localized and patchy opacity, rather than a homogenous one. Additionally, collapse usually doesn't cause significant mediastinal shift.

**Option C:** Pneumothorax, characterized by air accumulation in the pleural space, would typically show a hyperlucent area on the affected side, rather than homogenous opacity. Moreover, pneumothorax usually causes the mediastinum to shift away from the affected side.

**Option D:** Consolidation, which is a result of lung inflammation or infection, typically presents with patchy or segmental opacities rather than homogenous opacity. Consolidation also doesn't usually cause significant mediastinal shift.

**Clinical Pearl / High-Yield Fact**
When interpreting chest X-rays, remember that pleural effusions can be subtle and may not always be visible on an upright film. In such cases, a lateral decubitus film or a chest CT scan may be necessary to detect the effusion.

**✓ Correct Answer: B. Pleural effusion**