**Core Concept**
Long-acting beta 2 agonists (LABAs) are a class of medications used to treat asthma and chronic obstructive pulmonary disease (COPD). They work by stimulating beta 2 receptors in the lungs, leading to bronchodilation and relaxation of smooth muscle. This results in improved airflow and reduced symptoms of bronchospasm.

**Why the Correct Answer is Right**
Salmeterol is a LABA that has a slow onset of action but a long duration of effect, typically lasting 12 hours. This allows for once-daily dosing, making it a convenient option for patients. Salmeterol acts by binding to beta 2 receptors on the surface of bronchial smooth muscle cells, leading to the activation of adenylate cyclase and increased levels of cyclic AMP. This increases the activity of protein kinase A, which in turn relaxes the smooth muscle and leads to bronchodilation.

**Why Each Wrong Option is Incorrect**
**Option A:** Terbutaline is a short-acting beta 2 agonist, used for acute relief of bronchospasm. It has a rapid onset of action but a shorter duration of effect compared to LABAs like salmeterol.

**Option B:** Formoterol is a long-acting beta 2 agonist, but it has a faster onset of action compared to salmeterol, making it more suitable for patients who require rapid relief of bronchospasm.

**Option D:** Albuterol is a short-acting beta 2 agonist, used for acute relief of bronchospasm. It has a rapid onset of action but a shorter duration of effect compared to LABAs like salmeterol.

**Clinical Pearl / High-Yield Fact**
LABAs like salmeterol should not be used as monotherapy for asthma or COPD, but rather in combination with inhaled corticosteroids (ICS) to reduce the risk of exacerbations and improve lung function.

**Correct Answer:** C. Salmeterol