**Core Concept**
Heparin is a potent anticoagulant medication used to prevent and treat thromboembolic disorders. It works by activating antithrombin III, which in turn inhibits the coagulation cascade, particularly by inactivating thrombin and factor Xa.

**Why the Correct Answer is Right**
Heparin therapy is associated with several adverse effects, including heparin-induced thrombocytopenia (HIT), a prothrombotic condition caused by the formation of antibodies against platelet factor 4-heparin complexes. This leads to platelet activation, aggregation, and eventually, thrombocytopenia. Other common side effects of heparin include osteoporosis, alopecia, and a reversible elevation in liver enzymes. Heparin also increases the risk of bleeding, particularly in patients with pre-existing coagulopathy or those taking other anticoagulant medications.

**Why Each Wrong Option is Incorrect**
**Option A:** This is a potential side effect of heparin therapy, as it can cause a reversible elevation in liver enzymes.
**Option B:** This is a known complication of heparin therapy, as it can lead to a severe and potentially life-threatening thrombotic event.
**Option C:** This is a common side effect of heparin therapy, as it can cause a significant decrease in platelet count, increasing the risk of bleeding.

**Clinical Pearl / High-Yield Fact**
Heparin-induced thrombocytopenia (HIT) is a rare but serious side effect of heparin therapy, and it requires prompt recognition and treatment with alternative anticoagulants, such as argatroban or bivalirudin.

**Correct Answer: D**