## **Core Concept**
The question tests knowledge of radioactive antibodies used in the treatment of lymphoma, specifically those approved by the FDA. Radioactive antibodies, also known as radioimmunotherapy (RIT), are a form of targeted therapy that combines the specificity of monoclonal antibodies with the cytotoxicity of radiation. This approach aims to selectively destroy cancer cells while minimizing damage to normal tissues.

## **Why the Correct Answer is Right**
The correct answer, **B. Ibritomomab tiuxetan (Zevalin)**, is a monoclonal antibody labeled with the radioactive isotope Yttrium-90 (^90Y). It targets the CD20 antigen, which is predominantly found on the surface of B cells, making it effective against B-cell lymphomas. Ibritomomab tiuxetan is specifically FDA-approved for the treatment of relapsed or refractory low-grade or follicular B-cell non-Hodgkin's lymphoma (NHL). Its mechanism involves binding to CD20 on lymphoma cells, delivering a lethal dose of radiation that induces cell death.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **A. Rituximab** is a monoclonal antibody used in the treatment of certain types of lymphoma and does target the CD20 antigen, it is not radioactive and thus not classified as a radioactive antibody.
- **Option C:** **C. Alemtuzumab** targets the CD52 antigen on B and T cells and is used in the treatment of chronic lymphocytic leukemia (CLL) and cutaneous T-cell lymphoma. However, it is not a radioactive antibody.
- **Option D:** There might be confusion with other radioactive compounds or antibodies in development or with different indications, but **D.** is not specified here, making **B. Ibritomomab tiuxetan (Zevalin)** the standout correct choice.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **Ibritomomab tiuxetan (Zevalin)** is unique as it's specifically approved for lymphoma treatment and combines the specificity of a monoclonal antibody with the cytotoxic effect of radiation, making it a valuable option for targeted therapy in certain cases of NHL.

## **Correct Answer: B. Ibritomomab tiuxetan (Zevalin)**