**Core Concept**
The question is testing the ability to identify immunosuppressive agents that can cause profound myelosuppression, a potentially life-threatening side effect. Myelosuppression is characterized by a decrease in the production of blood cells, including red blood cells, white blood cells, and platelets.

**Why the Correct Answer is Right**
The correct answer is likely to be a drug that primarily targets the cell-mediated immune response through T-cell inhibition, without affecting bone marrow function. For example, **Cyclosporine** primarily inhibits calcineurin, which is involved in the activation of T-cells, but does not significantly affect the production of blood cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Drugs like **Cyclophosphamide** and **Busulfan** are known to cause myelosuppression as part of their mechanism of action. Cyclophosphamide is an alkylating agent that interferes with DNA replication, affecting rapidly dividing cells, including bone marrow cells. Busulfan is a chemotherapeutic agent that can cause prolonged myelosuppression.

**Option B:** **Azathioprine** is an antimetabolite that can cause myelosuppression by inhibiting the synthesis of nucleic acids.

**Option C:** **Mycophenolate Mofetil** is an inhibitor of inosine monophosphate dehydrogenase, which is involved in the synthesis of guanosine nucleotides. This can lead to myelosuppression.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that myelosuppression can be a dose-dependent side effect of many immunosuppressive agents. Monitoring of blood counts is crucial to prevent and manage this side effect.

**Correct Answer: D. Cyclosporine**