**Core Concept**
Heparin is an anticoagulant medication that inhibits the synthesis of thrombin and other coagulation factors by activating antithrombin III. Its reversal is crucial in situations of heparin-induced bleeding or during emergency surgery.

**Why the Correct Answer is Right**
The antidote of heparin is protamine sulfate, which binds to heparin molecules, forming a stable complex that inactivates heparin's anticoagulant activity. This binding reaction occurs rapidly, and protamine is effective in reversing heparin's effects within minutes. Protamine is a positively charged molecule that neutralizes the negatively charged heparin, thereby restoring normal coagulation pathways.

**Why Each Wrong Option is Incorrect**
**Option A:** Fibrinolytic agents like alteplase are used to break down clots, not reverse heparin's effects.
**Option B:** Vitamin K is an antidote for warfarin, a different type of anticoagulant that inhibits the production of clotting factors in the liver, not heparin.
**Option C:** Tranexamic acid is an antifibrinolytic agent used to prevent excessive bleeding, but it does not reverse heparin's anticoagulant effects.

**Clinical Pearl / High-Yield Fact**
When administering protamine to reverse heparin, it's essential to note that protamine can cause hypotension and anaphylactoid reactions in some patients. Therefore, slow infusion of protamine is recommended, and the patient should be monitored closely for signs of anaphylaxis.

**Correct Answer: D. Protamine sulfate**