**Core Concept**
Bronchodilators like short-acting beta-agonists (SABAs) work by relaxing bronchial smooth muscle, thereby increasing airflow and lung function. In reversible airway diseases such as asthma, this effect is temporary and often fully reversible with bronchodilator therapy.

**Why the Correct Answer is Right**
A 12% increase in forced expiratory volume in one second (FEV1) after administration of a SABA is indicative of reversible airway obstruction. This increase is due to the relaxation of bronchial smooth muscle caused by the beta-2 adrenergic receptor agonist effect of the SABA, which allows for improved airflow and lung function. The degree of reversibility is often used to diagnose and monitor asthma.

**Why Each Wrong Option is Incorrect**
**Option A:** A 50% decrease in FEV1 would indicate severe airway obstruction, which is not reversible with SABA therapy alone.
**Option C:** A 25% decrease in FEV1 after SABA administration would suggest partial reversibility, but the degree of reversibility is not as significant as seen in option B.
**Option D:** A 5% decrease in FEV1 is not a significant change and may not indicate reversible airway obstruction.

**Clinical Pearl / High-Yield Fact**
Reversible airway obstruction is a hallmark of asthma, and the degree of reversibility with bronchodilators like SABAs is a key diagnostic criterion. A 12% or greater increase in FEV1 after SABA administration is often used as a threshold to diagnose asthma.

✓ Correct Answer: B. Fev1 increase by 12% by SABA