**Core Concept**
The cell wall of Gram-negative bacteria contains several components, including lipopolysaccharides (LPS), peptidoglycan, and outer membrane proteins. Among these, lipopolysaccharides are known to elicit strong immune responses in animals, including humans.

**Why the Correct Answer is Right**
Endotoxic activity is primarily attributed to the lipid A moiety of lipopolysaccharides (LPS). Lipid A is a phosphorylated glycolipid that interacts with the Toll-like receptor 4 (TLR4) on immune cells, triggering a cascade of inflammatory responses. This interaction activates the myeloid differentiation factor 88 (MyD88) dependent pathway, leading to the production of pro-inflammatory cytokines. The lipid A moiety is responsible for the toxic effects of LPS, including fever, hypotension, and organ dysfunction.

**Why Each Wrong Option is Incorrect**
**Option A:** Peptidoglycan is a component of the cell wall of both Gram-positive and Gram-negative bacteria, but it does not possess endotoxic activity. It plays a structural role in maintaining the integrity of the cell wall.
**Option B:** Outer membrane proteins are essential for the function of the Gram-negative cell wall, but they do not contribute to endotoxic activity. They are involved in processes such as nutrient uptake and cell motility.
**Option C:** This option is incomplete and cannot be evaluated for correctness.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that the lipid A moiety of LPS is responsible for the endotoxic activity of Gram-negative bacteria. This knowledge is crucial for understanding the pathogenesis of sepsis and the development of targeted therapies.

**Correct Answer: B. Lipid A**