**Core Concept**
The MOPP chemotherapy regimen is a combination of four drugs used to treat Hodgkin's lymphoma. It consists of Mechlorethamine, Oncovin (Vincristine), Procarbazine, and Prednisone.

**Why the Correct Answer is Right**
The MOPP regimen is named after its four components, which work synergistically to target rapidly dividing cancer cells. Mechlorethamine is an alkylating agent that interferes with DNA replication, Oncovin (Vincristine) is a vinca alkaloid that inhibits microtubule formation, Procarbazine is an alkylating agent that cross-links DNA, and Prednisone is a corticosteroid that suppresses the immune system and reduces inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** Mechlorethamine is a correct component of the MOPP regimen.
**Option B:** Oncovin (Vincristine) is a correct component of the MOPP regimen.
**Option C:** Procarbazine is a correct component of the MOPP regimen.
**Option D:** Chlorambucil is not a component of the MOPP regimen; it is an alkylating agent used in other chemotherapy regimens.

**Clinical Pearl / High-Yield Fact**
The MOPP regimen is one of the first combination chemotherapy regimens used to treat Hodgkin's lymphoma, and it has been a cornerstone of treatment for this disease for many years. The addition of a fifth drug, Bleomycin, to the regimen resulted in the creation of the MOPP/ABV regimen, which is also used to treat Hodgkin's lymphoma.

**Correct Answer:** D. Chlorambucil.