**Core Concept**
The lineage-specific marker for B cells is a protein that is expressed on the surface of B cells and is used to identify and distinguish them from other cell types. This marker is critical in immunology and hematopathology for diagnosing B cell-related disorders.

**Why the Correct Answer is Right**
The correct answer is CD19, a transmembrane glycoprotein that is specifically expressed on the surface of B cells from the early stages of B cell development in the bone marrow until the mature B cell stage. CD19 plays a crucial role in B cell activation by providing a costimulatory signal to the B cell receptor complex, facilitating the activation of B cells in response to antigens. CD19 is also involved in the regulation of B cell development and differentiation.

**Why Each Wrong Option is Incorrect**
* **Option A:** CD3 is a marker for T cells, not B cells. It is a complex of three subunits that forms part of the T cell receptor complex on the surface of T cells.
* **Option B:** CD20 is another marker for B cells, but it is expressed on mature B cells and is involved in the regulation of B cell activation and proliferation. However, it is not the most specific marker for B cells.
* **Option C:** CD45 is a marker for all leukocytes, including B cells, T cells, and other white blood cells. It is a transmembrane glycoprotein that provides a common antigenic determinant for all leukocytes.

**Clinical Pearl / High-Yield Fact**
CD19 is a sensitive marker for B cell malignancies, including B cell acute lymphoblastic leukemia (B-ALL) and B cell lymphoma. It is often used in combination with other markers, such as CD20 and CD10, to diagnose and subtype B cell-related disorders.

**Correct Answer:** D.