**Core Concept**
The investigation of choice for evaluating pleural diseases involves assessing the pleural space for fluid, thickening, or nodules. This can be achieved through imaging modalities that provide detailed information about the pleural architecture.

**Why the Correct Answer is Right**
The correct answer is a chest CT scan, which is the investigation of choice for evaluating pleural diseases. This is because CT scans can provide detailed images of the pleural space, allowing for the detection of small pleural effusions, pleural thickening, and nodules. The high resolution of CT scans enables accurate characterization of pleural lesions and guides further management.

**Why Each Wrong Option is Incorrect**
**Option A:** Conventional chest radiograph (CXR) is not the investigation of choice for pleural diseases because it may not detect small pleural effusions or pleural thickening.
**Option B:** Ultrasonography can be useful in detecting pleural effusions, but it is not the investigation of choice for evaluating pleural diseases due to its limited ability to characterize pleural lesions.
**Option C:** MRI is not typically used for evaluating pleural diseases due to its limited availability and higher cost compared to CT scans.

**Clinical Pearl / High-Yield Fact**
A chest CT scan is essential in the evaluation of pleural diseases, as it can help guide thoracocentesis, pleural biopsy, or talc pleurodesis.

**Correct Answer:** C. MRI