**Core Concept**
Heparin is an anticoagulant that works by activating antithrombin III, which inhibits thrombin and factor Xa. An antidote for heparin is needed to reverse its anticoagulant effects in emergency situations such as bleeding or surgery.

**Why the Correct Answer is Right**
Protamine sulfate is a positively charged molecule that binds to the negatively charged heparin molecule, forming a stable complex that inactivates heparin's anticoagulant effects. This is because heparin has a high affinity for antithrombin III, and protamine displaces antithrombin III from the heparin molecule, thereby reversing the anticoagulant effects. Protamine also has a higher affinity for heparin than antithrombin III, making it a more effective antidote.

**Why Each Wrong Option is Incorrect**
* **Option A:** Vitamin K is an antidote for warfarin, not heparin. Warfarin inhibits vitamin K-dependent clotting factors, and vitamin K can reverse this effect by replenishing the necessary clotting factors.
* **Option B:** Fibrinolytics such as alteplase are used to break down clots, but they do not reverse the effects of heparin.
* **Option D:** Desmopressin is used to treat bleeding disorders such as hemophilia A and von Willebrand disease, but it is not an antidote for heparin.

**Clinical Pearl / High-Yield Fact**
Protamine sulfate is the only FDA-approved antidote for heparin, and it should be administered slowly to avoid anaphylactic reactions.

**Correct Answer:** C. Protamine sulfate