**Core Concept**
Acute inflammation involves the activation of endothelial cells, leading to changes in their cytoskeleton structure. This retraction of the cytoskeleton facilitates the movement of leukocytes through the endothelium, a process known as diapedesis.

**Why the Correct Answer is Right**
The retraction of endothelial cell cytoskeleton is mediated by the action of various enzymes, including phospholipase C (PLC) and protein kinase C (PKC). These enzymes activate the Rho kinase pathway, which leads to the contraction of endothelial cells and the subsequent diapedesis of leukocytes. This process allows for the migration of leukocytes from the bloodstream into the site of inflammation, where they can perform their immune functions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the retraction of endothelial cell cytoskeleton is not directly related to the contraction of smooth muscle cells.
**Option B:** This option is incorrect because the retraction of endothelial cell cytoskeleton does not involve the breakdown of the extracellular matrix.
**Option C:** This option is incorrect because the retraction of endothelial cell cytoskeleton is not a result of the activation of complement system.

**Clinical Pearl / High-Yield Fact**
The retraction of endothelial cell cytoskeleton is a critical step in the development of acute inflammation, allowing for the migration of leukocytes into the site of inflammation. Understanding this process is essential for the management of various inflammatory conditions.

**Correct Answer:** D. Leukocyte migration through the endothelium.