**Core Concept**
Sirolimus and cyclosporine are both immunosuppressive drugs used to prevent organ rejection in transplant patients. They work by inhibiting different pathways of the immune response. Sirolimus is a macrolide compound that inhibits the mammalian target of rapamycin (mTOR), a critical regulator of cell growth and proliferation.

**Why the Correct Answer is Right**
Sirolimus is more likely than cyclosporine to cause thrombocytopenia due to its mechanism of action. It inhibits the proliferation of platelets by blocking the mTOR pathway, leading to decreased platelet production in the bone marrow. In contrast, cyclosporine primarily inhibits T-cell activation, which is less likely to affect platelet production.

**Why Each Wrong Option is Incorrect**
* **Option A:** Sirolimus is not more likely than cyclosporine to cause hypertension. Both drugs can cause hypertension, but it is not a distinguishing feature between the two.
* **Option B:** Sirolimus is not more likely than cyclosporine to cause nephrotoxicity. While both drugs can cause nephrotoxicity, cyclosporine is actually more nephrotoxic than sirolimus.
* **Option C:** Sirolimus is not more likely than cyclosporine to cause hyperkalemia. Sirolimus can cause hyperkalemia, but it is not a distinguishing feature between the two drugs.

**Clinical Pearl / High-Yield Fact**
Sirolimus is a calcineurin-independent immunosuppressant, which makes it a good option for patients who are sensitive to calcineurin inhibitors like cyclosporine. However, sirolimus can cause hyperlipidemia and can be associated with an increased risk of lymphoma.

**Correct Answer:** D. Thrombocytopenia.