**Core Concept**
Theophylline is a methylxanthine derivative used in the management of asthma and chronic obstructive pulmonary disease (COPD). Its therapeutic effects are achieved through the inhibition of phosphodiesterase (PDE) enzymes, leading to increased levels of cyclic adenosine monophosphate (cAMP) within the airway smooth muscle cells, thereby causing bronchodilation.

**Why the Correct Answer is Right**
The therapeutic blood concentration of theophylline is a narrow range, typically between 8-20 mcg/mL. This range allows for optimal bronchodilation while minimizing the risk of toxicity. The mechanism of action of theophylline involves the inhibition of PDE enzymes, particularly PDE3 and PDE4, which are involved in the breakdown of cAMP. The increased levels of cAMP activate protein kinase A, leading to the relaxation of airway smooth muscle and bronchodilation.

**Why Each Wrong Option is Incorrect**
**Option A:** Less than 8 mcg/mL - This range is considered subtherapeutic, and patients may not experience optimal bronchodilation.
**Option B:** Greater than 20 mcg/mL - This range is considered toxic, and patients may experience adverse effects such as nausea, vomiting, and seizures.
**Option C:** 25-30 mcg/mL - This range is above the therapeutic window and increases the risk of toxicity.

**Clinical Pearl / High-Yield Fact**
It is essential to monitor theophylline blood levels regularly to ensure that they remain within the therapeutic range, as both underdosing and overdosing can occur.

**Correct Answer:** C. 25-30 mcg/mL is above the therapeutic window and increases the risk of toxicity