**Core Concept**
Radioimmunotherapy is a treatment approach that involves the use of radioactive antibodies to target and destroy cancer cells. In the context of lymphoma, this approach leverages the immune system to deliver radiation directly to the tumor site, minimizing damage to surrounding healthy tissues.

**Why the Correct Answer is Right**
The correct answer is Ibritumomab tiuxetan (Zevalin), a monoclonal antibody conjugated to the radioactive isotope Yttrium-90 (90Y). This antibody specifically targets the CD20 antigen present on the surface of B cells, which are often involved in lymphoma. When 90Y-ibritumomab tiuxetan binds to CD20, it delivers a lethal dose of radiation to the tumor cells, leading to their destruction.

**Why Each Wrong Option is Incorrect**
**Option A:** Lutetium-177 (177Lu) is a radioactive isotope used in some cancer treatments, but it is not the specific FDA-approved radioactive antibody for lymphoma treatment.

**Option B:** Brentuximab vedotin (Adcetris) is a monoclonal antibody-drug conjugate used to treat Hodgkin lymphoma and anaplastic large cell lymphoma, but it is not a radioactive antibody.

**Option C:** Tositumomab (Bexxar) is another monoclonal antibody used in radioimmunotherapy, but it is not the specific FDA-approved agent for lymphoma treatment mentioned in the question.

**Clinical Pearl / High-Yield Fact**
Radioimmunotherapy can be an effective treatment option for lymphoma, especially in patients who have failed conventional chemotherapy or radiation therapy. This approach allows for precise targeting of tumor cells, minimizing side effects and improving patient outcomes.

**Correct Answer:** C. Ibritumomab tiuxetan (Zevalin).