## **Core Concept**
Rituximab is a monoclonal antibody that targets the CD20 antigen, which is primarily found on the surface of B cells. It is used in the treatment of various conditions characterized by abnormal B cell function or proliferation. The mechanism of action involves complement-dependent cytotoxicity, antibody-dependent cellular cytotoxicity, and induction of apoptosis in B cells.

## **Why the Correct Answer is Right**
The correct answer involves conditions where B cell depletion is therapeutic. Rituximab is specifically used in the treatment of certain cancers, such as **non-Hodgkin's lymphoma (NHL)**, and autoimmune diseases like **rheumatoid arthritis (RA)**. It works by depleting B cells, which play a critical role in the pathogenesis of these diseases. The CD20 antigen is a target for therapy because it is expressed on most B cells, from the early pre-B cell stage until maturity, but not on stem cells or plasma cells.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Without the specific details of option A, it's not possible to directly address why it would be incorrect. However, if it does not relate to the use of rituximab in conditions like NHL or RA, it would be incorrect based on the established therapeutic applications of rituximab.
- **Option B:** Similarly, without specifics, if option B does not align with known indications for rituximab, such as certain types of lymphomas or autoimmune diseases, it would be considered incorrect.
- **Option C:** This option is not directly addressed but would be incorrect if it does not represent a condition treated with rituximab.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that rituximab can cause **infusion reactions** and increases the risk of **infections** due to B cell depletion. It's also crucial to note that patients treated with rituximab may have **negative B cell counts** and **low levels of immunoglobulins** for extended periods.

## **Correct Answer:** D. Non-Hodgkin's lymphoma.