## **Core Concept**
The core concept being tested here is the mechanism that prevents blood clotting in normal, intact blood vessels. This involves understanding the balance between pro-coagulant and anti-coagulant factors in the body.

## **Why the Correct Answer is Right**
The correct answer, **C. Endothelial cells secrete prostacyclin and nitric oxide**, is right because endothelial cells lining the blood vessels play a crucial role in preventing thrombosis. They secrete **prostacyclin**, which inhibits platelet aggregation, and **nitric oxide**, which not only inhibits platelet aggregation but also causes vasodilation. This environment prevents the activation of the coagulation cascade and subsequent clot formation in the absence of vascular injury.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, although **heparin** does have anticoagulant properties, the primary mechanism for preventing intravascular clotting is not through circulating heparin but through the endothelial cell surface and its products.
- **Option B:** This option is incorrect as **plasmin** and **fibrinolytic system** are involved in clot lysis rather than preventing clot formation. They play a role in dissolving clots that have already formed.
- **Option D:** This option is incorrect because **antithrombin III** does inhibit coagulation by inactivating thrombin and other clotting factors, but the question focuses on the normal state within intact vessels, where endothelial cell products are key.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the endothelium acts as a **thromboresistant surface** under normal conditions, primarily through the secretion of **prostacyclin** and **nitric oxide**. Damage to the endothelium disrupts this balance, leading to a prothrombotic state.

## **Correct Answer:** C. Endothelial cells secrete prostacyclin and nitric oxide.