**Core Concept**
B-cell markers are proteins or molecules expressed on the surface of B lymphocytes, which play a crucial role in the adaptive immune response. These markers help in identifying and differentiating B cells from other types of immune cells. Understanding B-cell markers is essential for diagnosing and managing various hematological disorders.

**Why the Correct Answer is Right**
The correct answer is likely to be a marker that is not specific to B cells. B cells express a variety of surface molecules, including CD19, CD20, CD21, and CD22, which are commonly used as markers for B-cell identification. These molecules are involved in B-cell development, activation, and proliferation. For example, CD19 is a transmembrane glycoprotein that plays a key role in B-cell development and activation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is a B-cell marker, specifically CD19, which is expressed on the surface of B cells and is involved in B-cell development and activation.
**Option B:** This option is also a B-cell marker, CD20, which is a transmembrane protein that is expressed on the surface of B cells and is involved in B-cell activation and proliferation.
**Option C:** This option is another B-cell marker, CD22, which is a transmembrane protein that is expressed on the surface of mature B cells and is involved in B-cell activation and signaling.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that B-cell markers can be used to diagnose and manage various hematological disorders, including B-cell lymphomas and leukemias. For example, CD20-positive B cells are a common target for therapeutic monoclonal antibodies, such as rituximab.

**Correct Answer:** A. CD19