**Core Concept**
The question is testing the knowledge of a nucleotide derivative that plays a crucial role in preventing organ rejection in transplant patients. This derivative is involved in the immune response and has a specific mechanism of action to prevent graft rejection.

**Why the Correct Answer is Right**
The correct answer is a nucleotide derivative that acts as an immunosuppressive agent. It works by inhibiting the activity of T-lymphocytes, which are a type of white blood cell that plays a key role in the immune response. This inhibition prevents the immune system from attacking the transplanted organ. The specific nucleotide derivative in question is a purine nucleoside analog that is phosphorylated to its active form, which then inhibits the enzyme inosine monophosphate dehydrogenase (IMPDH). This inhibition leads to a decrease in the production of guanosine nucleotides, which are essential for the activation and proliferation of T-lymphocytes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a nucleotide derivative that is commonly used in organ transplant patients.
**Option B:** This option is incorrect because it is not the correct mechanism of action for the nucleotide derivative in question.
**Option C:** This option is incorrect because it is not the correct nucleotide derivative that is used in organ transplant patients.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that immunosuppressive medications like the one in question require close monitoring of the patient's white blood cell count to prevent infections.

**Correct Answer:** C. Mycophenolate mofetil