**Core Concept**
Beta-2 agonists are bronchodilators that bind to beta-2 adrenergic receptors in the lungs, leading to smooth muscle relaxation and improved airflow. They are widely used in rescue therapy for acute bronchospasm in conditions like asthma and COPD.

**Why the Correct Answer is Right**
Ketotifen is a mast cell stabilizer, not a beta-2 agonist. It works by inhibiting histamine release and reducing inflammation, primarily used as a prophylactic agent in asthma, not for acute rescue. In contrast, formoterol, salbutamol, and bambuterol are all direct-acting beta-2 agonists with rapid onset and bronchodilatory effects, making them suitable for acute rescue therapy.

**Why Each Wrong Option is Incorrect**
Option A: Formoterol is a long-acting beta-2 agonist (LABA) with rapid onset, used in rescue and maintenance therapy.
Option B: Salbutamol is a short-acting beta-2 agonist (SABA) and the gold standard for acute asthma attacks.
Option C: Bambuterol is a short-acting beta-2 agonist with fast onset, used in rescue therapy for bronchospasm.
Option D: Ketotifen is **not** a beta-2 agonist; it is a mast cell stabilizer and is used preventively, not in acute rescue.

**Clinical Pearl / High-Yield Fact**
Remember: **SABA = Rescue**, **LABA = Maintenance**. Ketotifen is not a bronchodilator and should never be used in acute respiratory emergencies. Always check whether a drug is a beta-2 agonist or acts via a different mechanism.

✓ Correct Answer: D. Ketotifen