**Core Concept**
Cyclosporine is an immunosuppressant drug used to prevent organ rejection in transplant patients. Its mechanism of action involves the inhibition of calcineurin, a phosphatase enzyme crucial for the activation of T-lymphocytes. This results in a decrease in the production of interleukin-2 (IL-2), a cytokine essential for the proliferation and activation of T-cells.

**Why the Correct Answer is Right**
Cyclosporine binds to cyclophilin, a cytosolic receptor protein. The cyclosporine-cyclophilin complex then inhibits calcineurin, a phosphatase enzyme responsible for dephosphorylating and activating nuclear factor of activated T-cells (NFAT). The inhibition of calcineurin prevents the translocation of NFAT to the nucleus, where it would normally bind to DNA and initiate the transcription of IL-2. This results in a decrease in IL-2 production and a subsequent reduction in T-cell activation and proliferation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a correct description of the immunosuppressant action of cyclosporine. While cyclosporine does affect the immune system, its mechanism is not simply through the inhibition of IL-2 production.

**Option B:** This option is incorrect because it does not accurately describe the molecular mechanism of cyclosporine's immunosuppressant action. Cyclosporine does not directly inhibit the production of IL-2, but rather affects the signaling pathway that leads to IL-2 production.

**Option C:** This option is not a correct description of the immunosuppressant action of cyclosporine. While cyclosporine does affect T-cell activation, its mechanism is not simply through the inhibition of the CD28 receptor.

**Clinical Pearl / High-Yield Fact**
Cyclosporine is a calcineurin inhibitor, and its use is associated with an increased risk of nephrotoxicity and hypertension. This is due to the drug's effects on renal blood flow and glomerular filtration rate.

**Correct Answer: C. Inhibition of calcineurin.**