## **Core Concept**
Gemtuzumab ozogamicin is a monoclonal antibody used in the treatment of certain types of leukemia. It targets a specific antigen present on the surface of cancer cells. The drug is an example of antibody-dependent cellular cytotoxicity and works by inducing apoptosis in targeted cells.

## **Why the Correct Answer is Right**
The correct answer, **CD33**, is a surface antigen found on the majority of blast cells in acute myeloid leukemia (AML) and on some normal myeloid cells but not on lymphoid cells or normal hematopoietic stem cells. Gemtuzumab ozogamicin specifically binds to CD33, and this binding leads to the internalization of the antibody, which then releases a cytotoxic agent called calicheamicin. Calicheamicin induces DNA strand breaks, leading to cell death.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While some monoclonal antibodies target **CD20** (e.g., rituximab in non-Hodgkin lymphoma), gemtuzumab ozogamicin does not target this antigen.
- **Option B:** **CD22** is another target for certain monoclonal antibodies, particularly in chronic lymphocytic leukemia (CLL) and non-Hodgkin lymphoma, but not for gemtuzumab ozogamicin.
- **Option D:** **CD30** is targeted by brentuximab vedotin in Hodgkin lymphoma and some types of non-Hodgkin lymphoma, not by gemtuzumab ozogamicin.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that gemtuzumab ozogamicin was initially approved for the treatment of relapsed or refractory AML but has seen its use evolve with changing guidelines and evidence. Monitoring for infusion-related reactions and veno-occlusive disease (especially when used with other hepatotoxic agents) is crucial.

## **Correct Answer:** C. CD33.