**Core Concept**
Thrombocytopenia is a condition characterized by low platelet count in the blood, which can increase the risk of bleeding. It can be caused by various factors, including certain medications.

**Why the Correct Answer is Right**
Thrombocytopenia is a known side effect of heparin, a medication commonly used as an anticoagulant to prevent blood clot formation. Heparin-induced thrombocytopenia (HIT) occurs due to the formation of antibodies against the platelet factor 4-heparin complex, which activates platelet destruction. This results in a decrease in platelet count, increasing the risk of bleeding complications.

**Why Each Wrong Option is Incorrect**
* **Option A:** Although aspirin is an antiplatelet agent, it primarily inhibits platelet aggregation rather than causing thrombocytopenia.
* **Option B:** Warfarin is an anticoagulant, but its primary mechanism of action is through the inhibition of vitamin K-dependent clotting factors, not platelet destruction.
* **Option C:** Cytotoxic chemotherapy can cause thrombocytopenia, but it is not a direct side effect of a specific medication like heparin.
* **Option D:** Antibiotics, such as penicillins, are not commonly associated with thrombocytopenia.

**Clinical Pearl / High-Yield Fact**
Thrombocytopenia due to heparin can be a life-threatening complication, and prompt recognition and discontinuation of heparin are essential to prevent further platelet destruction and bleeding.

**Correct Answer:** A. Heparin.