**Core Concept**
Acute inflammation is a complex process mediated by various chemical signals, including cytokines, chemokines, and other inflammatory mediators. These mediators can be derived from cells or be pre-formed in the body.

**Why the Correct Answer is Right**
The correct answer is histamine, which is a pre-formed mediator stored in mast cells. When mast cells are activated, they release histamine, leading to increased vascular permeability, smooth muscle contraction, and mucous production. Histamine exerts its effects through H1 and H2 receptors, causing vasodilation and increased mucus secretion.

**Why Each Wrong Option is Incorrect**
**Option A:** Cytokines are cell-derived mediators of inflammation, released in response to tissue damage or infection. They play a crucial role in coordinating the inflammatory response.
**Option B:** Prostaglandins are cell-derived mediators of inflammation, produced by the enzyme cyclooxygenase (COX) from arachidonic acid. They mediate pain, fever, and inflammation.
**Option C:** Leukotrienes are cell-derived mediators of inflammation, produced by the enzyme 5-lipoxygenase from arachidonic acid. They mediate bronchoconstriction and increased mucus production.

**Clinical Pearl / High-Yield Fact**
Remember that histamine is a pre-formed mediator stored in mast cells, which can be released immediately in response to an allergic reaction or anaphylaxis.

**Correct Answer:** D. Histamine.