**Core Concept**
Reversible airway obstruction is a characteristic feature of asthma and chronic obstructive pulmonary disease (COPD), indicating that the airway narrowing can be reversed with appropriate treatment. In this context, the forced expiratory volume in 1 second (FEV1) is a critical measure of lung function.

**Why the Correct Answer is Right**
A reversible airway obstruction is defined by a significant increase in FEV1 after administration of an inhaled short-acting beta-2 agonist (SABA). This increase is due to the relaxation of bronchial smooth muscle, allowing for easier airflow. The SABA stimulates beta-2 receptors in the airway smooth muscle, leading to their relaxation and subsequent increase in FEV1. A 12-15% increase in FEV1 is generally considered indicative of reversible airway obstruction.

**Why Each Wrong Option is Incorrect**
**Option A:** A 5% increase in FEV1 may not be significant enough to distinguish reversible airway obstruction from other conditions.
**Option B:** A 20% increase in FEV1 might be seen in some patients with asthma or COPD, but it is not the specific threshold used to define reversible airway obstruction.
**Option C:** A decrease in FEV1 after SABA administration would indicate an irreversible airway obstruction or another condition, such as bronchospasm.
**Option D:** A 0% increase in FEV1 would indicate no change in airway obstruction, which is not consistent with reversible airway obstruction.

**Clinical Pearl / High-Yield Fact**
When assessing lung function in patients with asthma or COPD, a 12-15% increase in FEV1 after SABA administration is a key indicator of reversible airway obstruction. This can help guide treatment decisions and monitor disease progression.

**Correct Answer:** B. 12-15%