**Core Concept**
The question requires an understanding of **2nd generation antihistamines**, which are preferred over 1st generation antihistamines due to their **selective peripheral H1 receptor antagonism**, resulting in fewer **central nervous system (CNS) side effects**. This class of drugs is used to treat **allergic rhinitis**, **urticaria**, and other **allergic conditions**.

**Why the Correct Answer is Right**
Desloratadine is a **non-sedating, long-acting H1 receptor antagonist** with a high affinity for the **H1 receptor**, making it a potent option for relieving **allergic symptoms**. Its mechanism involves competitive inhibition of **histamine** at the **H1 receptor site**, thereby reducing **vasodilation**, **smooth muscle contraction**, and **increased vascular permeability** associated with **allergic reactions**. Desloratadine has been shown to be effective in reducing symptoms of **allergic rhinitis** and **chronic idiopathic urticaria**.

**Why Each Wrong Option is Incorrect**
**Option A:** Astemizole, although a 2nd generation antihistamine, has been withdrawn from many markets due to concerns over **cardiac arrhythmias**, specifically **QT interval prolongation**.
**Option C:** Promethazine is a 1st generation antihistamine with significant **sedative effects** due to its ability to cross the **blood-brain barrier** and act on the **CNS**.
**Option D:** Fexofenadine is another 2nd generation antihistamine but is generally considered less potent than Desloratadine in terms of **H1 receptor affinity** and **duration of action**.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that **2nd generation antihistamines** like Desloratadine are preferred for long-term use due to their favorable **side effect profile** and **lack of sedation**, making them safer for patients who require **prolonged antihistamine therapy**.

**✓ Correct Answer: B. Desloratadine**