**Core Concept**
Increased permeability in acute inflammation is a key event that allows the passage of fluid, white blood cells, and other substances from the blood vessels into the interstitial space, leading to edema and tissue damage.

**Why the Correct Answer is Right**
The correct answer involves the contraction of endothelial cells, which leads to the formation of gaps between them, increasing vascular permeability. This is mediated by the release of various chemical mediators, such as histamine and bradykinin, which stimulate the contraction of endothelial cells and the opening of gap junctions between them. The increased permeability is also facilitated by the breakdown of the endothelial glycocalyx, which normally maintains the integrity of the endothelial barrier.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the increased permeability in acute inflammation is not primarily due to the contraction of smooth muscle cells, which would lead to vasoconstriction rather than increased permeability.
**Option B:** This option is incorrect because the increased permeability in acute inflammation is not primarily due to the increased hydrostatic pressure within the blood vessels, which would lead to increased filtration but not necessarily increased permeability.
**Option C:** This option is incorrect because the increased permeability in acute inflammation is not primarily due to the breakdown of the basement membrane, which would lead to a more chronic and destructive process.

**Clinical Pearl / High-Yield Fact**
In acute inflammation, the increased permeability is a reversible process that can be modulated by various therapeutic agents, such as corticosteroids and non-steroidal anti-inflammatory drugs (NSAIDs), which can inhibit the release of chemical mediators and reduce vascular permeability.

**Correct Answer: D. Increased contraction of endothelial cells and opening of gap junctions.**