**Core Concept**
The process of immediate sustained permeability in cells refers to the rapid increase in the passage of substances through cell membranes, leading to increased vascular permeability. This phenomenon is often observed in response to tissue injury or inflammation.

**Why the Correct Answer is Right**
Direct injury and necrosis of cells lead to the disruption of the cell membrane, causing an influx of ions, water, and other substances into the interstitial space. This results in increased vascular permeability, allowing the passage of large molecules such as proteins and even cells through the endothelial layer. The mechanism underlying this process is often referred to as "cytotoxic edema." In the context of direct injury and necrosis, the cell membrane is damaged, disrupting the normal selective permeability that is maintained by the endothelial layer.

**Why Each Wrong Option is Incorrect**
**Option B:** Histamine is a vasodilator that can increase vascular permeability, but it does so through a slower, more sustained process involving the contraction of endothelial cells and the opening of gaps between them. This is not an immediate, sustained increase in permeability.
**Option C:** Serotonin can also increase vascular permeability, but like histamine, it does so through a slower process involving the contraction of endothelial cells and the opening of gaps between them.
**Option D:** Leucocyte-mediated endothelial injury is a process that can lead to increased vascular permeability, but it occurs over a longer period and involves the activation of leucocytes and the release of various inflammatory mediators.

**Clinical Pearl / High-Yield Fact**
In the context of tissue injury or inflammation, the immediate sustained increase in vascular permeability is a critical step in the development of edema. It is essential to recognize that the mechanisms underlying this process can vary depending on the underlying cause, and that different mediators can contribute to increased permeability in different contexts.

**✓ Correct Answer: A. Direct injury and necrosis**