**Core Concept**
Zidovudine is an antiretroviral medication used to treat HIV/AIDS. It works by inhibiting the replication of HIV through its active metabolite, 3'-azido-3'-deoxythymidine triphosphate (AZT-TP), which competes with thymidine triphosphate (dTTP) for incorporation into viral DNA. However, zidovudine can also cause bone marrow suppression, leading to anemia and neutropenia.

**Why the Correct Answer is Right**
The correct answer is likely to be a medication that also causes bone marrow suppression or interferes with hematopoiesis. This would lead to additive effects when administered to a patient already taking zidovudine.

* The correct answer, **Ganciclovir**, is an antiviral medication used to treat cytomegalovirus (CMV) infections. It works by inhibiting viral DNA synthesis through its incorporation into the viral genome. However, ganciclovir also causes bone marrow suppression, particularly affecting the production of neutrophils and platelets.
* **Option A:** This is likely to be a medication that does not cause bone marrow suppression or interfere with hematopoiesis. For example, **Ritonavir** is a protease inhibitor used to treat HIV/AIDS, but it does not cause bone marrow suppression.
* **Option B:** This could be a medication that causes bone marrow suppression, but it is not as likely to cause additive effects with zidovudine as ganciclovir. For example, **Methotrexate** is an immunosuppressant used to treat cancer and autoimmune diseases, but it primarily affects the production of red and white blood cells.
* **Option C:** This could be a medication that does not cause bone marrow suppression or interfere with hematopoiesis. For example, **Lamivudine** is an antiretroviral medication used to treat HIV/AIDS, but it does not cause bone marrow suppression.

**Clinical Pearl / High-Yield Fact**
When administering antiviral medications to patients with HIV/AIDS, it is essential to consider potential interactions and additive effects on hematopoiesis. This includes monitoring for signs of anemia and neutropenia, particularly when combining medications that cause bone marrow suppression.

**Correct Answer: D. Ganciclovir**