## Core Concept
The question tests the knowledge of immunosuppressant drugs, which are used to prevent rejection in organ transplantation and to treat autoimmune diseases. Immunosuppressants can be broadly classified into several categories, including corticosteroids, calcineurin inhibitors, antiproliferative agents, and monoclonal antibodies.

## Why the Correct Answer is Right
Azathioprine is indeed an immunosuppressant. It works by inhibiting purine synthesis, which affects the proliferation of T and B lymphocytes. Mycophenolate mofetil, another immunosuppressant, acts by inhibiting inosine monophosphate dehydrogenase, an enzyme critical for lymphocyte proliferation. Cyclophosphamide is also an immunosuppressant, used in the treatment of certain autoimmune diseases and as a conditioning regimen for bone marrow transplantation. It works by cross-linking DNA strands, thereby inhibiting DNA synthesis and function. On the other hand, is not typically classified as an immunosuppressant but rather as an antibiotic.

## Why Each Wrong Option is Incorrect
* **Option A:** Azathioprine is an immunosuppressant.
* **Option B:** Mycophenolate mofetil is an immunosuppressant.
* **Option D:** Cyclophosphamide is an immunosuppressant.

## Clinical Pearl / High-Yield Fact
A key point to remember is that while many drugs have immunosuppressive effects, not all are primarily used for this purpose. For example, certain antibiotics can have immunosuppressive properties but are not classified as immunosuppressants.

## Correct Answer: .