**Core Concept**
Thromboxanes and leukotrienes are types of eicosanoids that play significant roles in vascular regulation and inflammation. These molecules are synthesized from arachidonic acid through the action of cyclooxygenase (COX) enzymes and lipoxygenase enzymes, respectively.

**Why the Correct Answer is Right**
Leukotrienes, specifically LTB4 and LTC4, LTC5, and LTD4, are known to cause vasoconstriction and increase vascular permeability. This is mediated through the binding of leukotrienes to their receptors on smooth muscle cells and endothelial cells. The activation of these receptors triggers a cascade of signaling pathways that lead to the contraction of smooth muscle cells and the increase in vascular permeability. Thromboxane A2 (TXA2) also causes vasoconstriction, but it does not increase vascular permeability.

**Why Each Wrong Option is Incorrect**
**Option A:** Prostaglandins, such as PGE2 and PGF2α, cause vasodilation and increase vascular permeability, but they do not cause vasoconstriction. Prostaglandins are involved in various physiological and pathological processes, including inflammation, fever, and pain.
**Option B:** Platelet-activating factor (PAF) causes platelet activation, aggregation, and vasoconstriction, but it does not increase vascular permeability. PAF is involved in the regulation of platelet function and inflammation.
**Option C:** Prostacyclin (PGI2) causes vasodilation and inhibits platelet aggregation, but it does not increase vascular permeability. Prostacyclin is involved in the regulation of vascular tone and platelet function.

**Clinical Pearl / High-Yield Fact**
Eicosanoids are involved in various physiological and pathological processes, including inflammation, pain, fever, and vascular regulation. Understanding the mechanisms of action of these molecules is crucial for the development of new therapeutic strategies for the treatment of diseases such as asthma, cardiovascular disease, and cancer.

**Correct Answer:** C. Thromboxane A2 causes vasoconstriction but does not increase vascular permeability, while leukotrienes cause both vasoconstriction and increase vascular permeability.