**Question:** Which is not a 2nd generation anti-histamine agents

**Core Concept:** 2nd generation antihistamines are a class of drugs that were developed to overcome the side effects associated with first-generation antihistamines, such as anticholinergic effects. These newer agents are selective for histamine H1 receptors and have fewer side effects.

**Why the Correct Answer is Right:**

The correct answer, **C. Cetirizine**, is a specific example of a second-generation antihistamine. It selectively binds to the H1 receptors without affecting other types of receptors, thus minimizing the side effects associated with first-generation antihistamines.

**Why Each Wrong Option is Incorrect:**

A. **Desloratadine (Option A)** is another selective H1 receptor antagonist and thus is not considered a second-generation antihistamine.
B. **Loratadine (Option B)** is also a selective H1 receptor antagonist and falls under the category of second-generation antihistamines.
D. **Promethazine (Option D)** is a typical first-generation antihistamine that also possesses additional effects due to its interaction with other receptors, making it a second-generation antihistamine.

**Clinical Pearl / High-Yield Fact:**

Understanding the difference between first- and second-generation antihistamines is crucial for selecting the appropriate treatment for patients with allergic symptoms. Second-generation antihistamines are generally safer and cause fewer side effects, making them the preferred choice in many clinical situations.

**Correct Answer:** C. Cetirizine

**Correct Answer Explanation:** Cetirizine is a selective H1 receptor antagonist, meaning it specifically binds to H1 receptors without affecting other types of receptors. This selective action contributes to its reduced side effects compared to first-generation antihistamines, making cetirizine a representative of the second-generation antihistamines.