**Core Concept**
Increased capillary permeability is a critical component of the inflammatory response, allowing fluid, proteins, and white blood cells to escape from the bloodstream into the interstitial space. This phenomenon is mediated by various chemical mediators and cellular factors that disrupt the normal integrity of the endothelial cells lining the capillaries.

**Why the Correct Answer is Right**
Increased capillary permeability is a hallmark of inflammation and is primarily caused by the release of chemical mediators such as histamine, bradykinin, and leukotrienes. These substances activate endothelial cells to increase the expression of adhesion molecules, leading to the adhesion and transmigration of leukocytes across the endothelial layer. Additionally, the release of nitric oxide and other vasodilators causes vasodilation, further increasing the permeability of capillaries.

**Why Each Wrong Option is Incorrect**
**Option A:** Vasoconstriction caused by sympathetic nervous system activation (e.g., during stress or anxiety) can actually decrease capillary permeability, making it an incorrect option. Vasoconstriction reduces blood flow and increases vascular resistance, which can mitigate the effects of increased capillary permeability.

**Option B:** Ischemia or hypoxia can increase capillary permeability by releasing chemical mediators such as adenosine and ATP, which stimulate endothelial cells to increase permeability. Therefore, this option is incorrect as it can contribute to increased capillary permeability.

**Option D:** Direct injury to the endothelial cells can also increase capillary permeability by disrupting the normal integrity of the endothelial layer. This can lead to the loss of tight junctions and increased permeability, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that increased capillary permeability is a reversible process, and the resolution of inflammation is often accompanied by a return to normal capillary permeability. This knowledge can be useful in clinical settings, where understanding the pathophysiology of inflammation is crucial for managing various diseases.

**Correct Answer: A. Vasoconstriction caused by sympathetic nervous system activation can decrease capillary permeability, making it the correct answer.**