**Core Concept**
Mesothelioma and adenocarcinoma are two distinct types of malignant tumors that can arise in the pleura or peritoneum. Differentiating between these two types is crucial for proper diagnosis and treatment planning. Immunohistochemistry plays a vital role in this differentiation, where specific markers are used to identify the tumor type.

**Why the Correct Answer is Right**
The correct marker for differentiating between mesothelioma and adenocarcinoma is Calretinin. Calretinin is a calcium-binding protein that is highly expressed in mesothelial cells. In immunohistochemical analysis, Calretinin positivity is seen in >70% of mesothelioma cases, whereas adenocarcinoma shows weak or negative staining. This is because mesothelioma cells retain the expression of Calretinin, whereas adenocarcinoma cells lose it.

**Why Each Wrong Option is Incorrect**
**Option A:** WT1 (Wilms Tumor 1) is a marker that is positive in mesothelioma but also shows significant overlap with adenocarcinoma, making it a less reliable marker for differentiation.

**Option B:** BerEP4 is a marker that is positive in adenocarcinoma but also shows weak positivity in mesothelioma, making it an unreliable marker for differentiation.

**Option C:** CEA (Carcinoembryonic Antigen) is a marker that is positive in both mesothelioma and adenocarcinoma, making it a non-specific marker for differentiation.

**Clinical Pearl / High-Yield Fact**
Calretinin positivity can be seen in various other conditions, including reactive mesothelial proliferations and some types of ovarian tumors. Therefore, it is essential to use Calretinin in combination with other markers to confirm the diagnosis of mesothelioma.

**Correct Answer:** D. Calretinin