**Core Concept**
Endotoxic shock is a life-threatening medical condition caused by the body's response to an overwhelming infection with Gram-negative bacteria. This response is triggered by the release of endotoxins, also known as lipopolysaccharides, from the bacterial cell wall. The endotoxins bind to specific receptors on immune cells, initiating a cascade of inflammatory and immune responses that can lead to organ dysfunction and failure.

**Why the Correct Answer is Right**
The correct answer is the release of endotoxins from Gram-negative bacteria. When these bacteria undergo lysis or are killed by the host's immune system, they release endotoxins into the bloodstream. These endotoxins then bind to the CD14 receptor on the surface of macrophages and other immune cells, which activates the release of pro-inflammatory cytokines such as TNF-alpha and IL-1 beta. These cytokines stimulate the production of other inflammatory mediators, leading to the systemic inflammatory response syndrome (SIRS) characteristic of endotoxic shock.

**Why Each Wrong Option is Incorrect**
**Option A:** While the immune system does play a crucial role in the development of endotoxic shock, it is not the primary trigger for the condition. The correct answer is the release of endotoxins, which activates the immune response.
**Option B:** Complement system activation is a secondary response to endotoxin exposure, but it is not the primary initiator of endotoxic shock.
**Option C:** While the release of histamine and other vasoactive mediators contributes to the pathophysiology of endotoxic shock, it is not the primary trigger for the condition.

**Clinical Pearl / High-Yield Fact**
Endotoxic shock is often characterized by a triad of symptoms: hypotension, tachycardia, and oliguria. This triad is often referred to as the "endotoxin triad" and is a key clinical clue to the diagnosis of endotoxic shock.

**Correct Answer: D. Release of endotoxins from Gram-negative bacteria.**