**Core Concept**
Endothelial retraction in acute inflammation is a crucial step that leads to increased permeability of blood vessels. This phenomenon is closely related to the histopathological changes observed in acute inflammation, which involves the disruption of the endothelial lining of blood vessels.

**Why the Correct Answer is Right**
The correct answer is C. Delayed prolonged increase in permeability. This occurs due to the contraction and retraction of endothelial cells, which exposes the underlying basement membrane and increases the gap between adjacent endothelial cells. This increased gap allows for the free passage of fluid, proteins, and white blood cells into the interstitial space, leading to edema and further exacerbating the inflammatory response. The delayed onset of increased permeability is due to the time required for endothelial cell contraction and retraction, which can take several hours after the initial insult.

**Why Each Wrong Option is Incorrect**
**Option A:** A delayed transient increase in permeability is incorrect because the increase in permeability is not transient in nature but rather prolonged. The endothelial retraction leads to a sustained increase in permeability, which is a hallmark of acute inflammation.

**Option B:** An immediate transient increase in permeability is incorrect because the increase in permeability is not immediate. It takes several hours for endothelial cell contraction and retraction to occur, leading to increased permeability. Additionally, the increase in permeability is not transient but rather prolonged.

**Option D:** An immediate transient decrease in permeability is incorrect because endothelial retraction leads to an increase in permeability, not a decrease. The contraction and retraction of endothelial cells expose the underlying basement membrane, leading to increased permeability.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that endothelial retraction is a key event in acute inflammation that leads to increased permeability and edema. This process is closely related to the histopathological changes observed in acute inflammation and is a critical factor in the development of edema and further exacerbating the inflammatory response.

**✓ Correct Answer: C. Delayed prolonged increase in permeability**