**Core Concept**
B cells are lymphocytes that differentiate into plasma cells and produce antibodies. Key surface markers on B cells include CD19, CD20, and CD10, which are involved in B cell development, activation, and function. CD134 (also known as OX40) is not a specific B cell marker and is instead expressed on activated T cells and some dendritic cells.

**Why the Correct Answer is Right**
CD134 (OX40) is a costimulatory receptor primarily expressed on activated T cells, not B cells. It plays a role in T cell proliferation and survival. CD19, CD20, and CD10 are all recognized as B cell lineage markers: CD19 is expressed early in B cell development, CD20 is present on mature B cells, and CD10 is a marker of early B cell precursors. Thus, CD134 does not belong to the B cell marker group.

**Why Each Wrong Option is Incorrect**
Option A: CD19 is a core B cell marker expressed from pre-B to mature B cells and is used in B cell identification in flow cytometry.
Option B: CD20 is a key B cell surface antigen used in therapies like rituximab and is consistently expressed on B cells.
Option D: CD10 is a marker of early B cell development and is expressed on pro-B and pre-B cells.

**Clinical Pearl / High-Yield Fact**
In immunophenotyping, CD19, CD20, and CD10 are essential for diagnosing B cell lymphomas, while CD134 is exclusively a T cell marker — remember: **"CD134 = T cell, not B cell."**

✓ Correct Answer: C. CD 134