**Core Concept**
Endotoxin shock, also known as septic shock, is a severe systemic inflammatory response syndrome (SIRS) caused by the release of endotoxins from Gram-negative bacteria. Endotoxins are lipopolysaccharides (LPS) that are components of the outer membrane of Gram-negative bacteria.

**Why the Correct Answer is Right**
Endotoxin shock is initiated when endotoxins are released from the bacterial cell wall during bacterial lysis or cell death. The endotoxins then bind to specific receptors on the surface of host immune cells, such as macrophages and endothelial cells, triggering a cascade of inflammatory responses. This leads to the release of pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 beta (IL-1β), which cause vascular dysfunction, hypotension, and organ failure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because endotoxin shock is not initiated by exotoxins, which are proteins secreted by bacteria.
**Option B:** This option is incorrect because endotoxin shock is not initiated by the release of histamine from mast cells, although histamine can contribute to the vasodilation and hypotension seen in septic shock.
**Option C:** This option is incorrect because endotoxin shock is not initiated by the release of bradykinin, although bradykinin can also contribute to vasodilation and hypotension in septic shock.

**Clinical Pearl / High-Yield Fact**
Mnemonic: "SEPSIS" - S: Systemic inflammatory response syndrome, E: Endotoxin release, P: Pro-inflammatory cytokine release, S: Sepsis, I: Infection, S: Shock.

**Correct Answer: A. Endotoxins from Gram-negative bacteria.**