**Core Concept**
Cyclosporine is a calcineurin inhibitor that suppresses T-cell activation by inhibiting the production of interleukin-2 (IL-2). This immunosuppressive action is crucial in preventing organ rejection in renal transplant patients.

**Why the Correct Answer is Right**
Cyclosporine's mechanism of action involves binding to cyclophilin, which then inhibits calcineurin. This inhibition prevents the dephosphorylation of the nuclear factor of activated T-cells (NFAT), thereby blocking the transcription of IL-2. As a result, T-cell activation and proliferation are reduced, leading to immunosuppression. The most significant side effect associated with this mechanism is nephrotoxicity, which can manifest as acute or chronic kidney injury.

**Why Each Wrong Option is Incorrect**
**Option A:** Hypokalemia is not a direct side effect of cyclosporine. Although electrolyte imbalances can occur, hypokalemia is more commonly associated with other medications like diuretics.

**Option B:** Hirsutism is a side effect of cyclosporine, but it is not the most significant or major side effect. It is more of a cosmetic concern.

**Option C:** Gastrointestinal upset, such as nausea and vomiting, can occur with cyclosporine, but they are not the most significant side effects.

**Clinical Pearl / High-Yield Fact**
Remember that cyclosporine's nephrotoxicity can be dose-dependent and may be exacerbated by other nephrotoxic agents. Monitor renal function closely in patients receiving cyclosporine.

**Correct Answer:** B. Hirsutism. Hirsutism is a major side effect of cyclosporine, characterized by excessive hair growth, particularly on the face and body.