**Core Concept**
Promethazine is a first-generation antihistamine that exhibits significant anticholinergic activity due to its ability to cross the blood-brain barrier and bind to muscarinic receptors. This is in contrast to second-generation antihistamines, which are designed to be more selective for peripheral H1 receptors and have reduced central nervous system penetration.

**Why the Correct Answer is Right**
Promethazine's anticholinergic effects are responsible for its side effects, such as dry mouth, blurred vision, and constipation. These effects are mediated by its ability to block muscarinic acetylcholine receptors in the central and peripheral nervous systems. The other options, Astemizole, Levocetirizine, and Loratadine, are second-generation antihistamines that are designed to minimize central nervous system penetration and anticholinergic activity.

**Why Each Wrong Option is Incorrect**
**Option B:** Astemizole is a second-generation antihistamine that lacks significant anticholinergic activity due to its limited central nervous system penetration. Its side effect profile is more favorable than that of first-generation antihistamines like Promethazine.
**Option C:** Levocetirizine is a second-generation antihistamine that is highly selective for peripheral H1 receptors and has minimal anticholinergic activity. Its side effect profile is similar to that of other second-generation antihistamines.
**Option D:** Loratadine is also a second-generation antihistamine that lacks significant anticholinergic activity due to its limited central nervous system penetration. It is known for its long half-life and once-daily dosing regimen.

**Clinical Pearl / High-Yield Fact**
When choosing an antihistamine, it is essential to consider the patient's age, medical conditions, and potential drug interactions. First-generation antihistamines like Promethazine may be more effective for treating allergic reactions, but they are associated with significant anticholinergic side effects. Second-generation antihistamines like Astemizole, Levocetirizine, and Loratadine are generally safer and more suitable for patients with certain medical conditions.

**✓ Correct Answer: A. Promethazine**