A 25-year-old machinist is injured by a metal sliver in his left hand. Over the next few days, the wounded area becomes reddened, tender, swollen, and feels warm to the touch. The patient goes to the emergency room to have the sliver removed. Twenty-four hours later, endothelial cells at the site of injury in the patient release a chemical mediator that inhibits further platelet aggregation. Name this mediator of inflammation.

**Core Concept:** Inflammation is a protective response of the body to eliminate the cause of injury, remove necrotic tissue, and facilitate tissue repair. Inflammatory mediators are chemical substances released by the immune cells at the site of injury.

**Why the Correct Answer is Right:** The correct answer is D-histamine. Histamine is a chemical mediator released by mast cells and basophils during inflammation. It plays a crucial role in increasing vascular permeability, causing vasodilation and increased blood flow to the affected area, which contributes to redness and warmth. Additionally, histamine activates pain receptors, contributing to pain and swelling. Platelet aggregation inhibition is not a direct effect of histamine, but rather a result of the increased blood flow that allows more immune cells to reach the site of injury, including those that release substances that inhibit platelet aggregation.

**Why Each Wrong Option is Incorrect:**

A. **Leukotrienes:** These are lipid mediators mainly involved in bronchoconstriction and increased vascular permeability, not platelet aggregation inhibition.

B. **Prostaglandins:** Prostaglandins are lipid mediators involved in pain, fever, and increased vascular permeability, but not platelet aggregation inhibition.

C. **Serotonin:** Serotonin contributes to increased vascular permeability and pain, but not platelet aggregation inhibition.

**Clinical Pearl:** Understanding the role of histamine in inflammation is essential for clinical practice, as histamine-blocking drugs are used in allergic reactions, migraine, and certain dermatological conditions. However, they can also have side effects like flushing, hypotension, and sometimes bronchoconstriction due to non-selective blockade of H1 and H2 receptors.

✓ Correct Answer: B. Prostaglandin (PGI2)