**Core Concept**
Theophylline is a methylxanthine derivative with bronchodilatory, anti-inflammatory, and cardiovascular effects, used in the management of asthma and chronic obstructive pulmonary disease (COPD). It acts as a phosphodiesterase inhibitor, which increases the intracellular concentration of cyclic adenosine monophosphate (cAMP) in airway smooth muscle cells.

**Why the Correct Answer is Right**
Theophylline works by inhibiting the phosphodiesterase enzyme, which breaks down cAMP. This leads to an increase in cAMP levels, causing relaxation of airway smooth muscle and bronchodilation. Additionally, theophylline has anti-inflammatory properties, which contribute to its therapeutic effects in asthma and COPD.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because theophylline is not a selective beta-2 adrenergic receptor agonist, unlike salbutamol or terbutaline. Theophylline's mechanism of action is distinct from beta-2 agonists.
**Option B:** Not enough information is provided to assess this option, but theophylline is known to have a narrow therapeutic index and requires regular monitoring of serum levels to avoid toxicity.
**Option C:** This option is incorrect because theophylline is actually known to have a significant effect on the cardiovascular system, including vasodilation and decreased systemic vascular resistance, but it is not typically used as a primary treatment for hypertension.

**Clinical Pearl / High-Yield Fact**
Theophylline is a narrow therapeutic index medication, and its plasma concentrations should be monitored regularly to avoid toxicity, which can manifest as nausea, vomiting, and seizures at high levels.

**Correct Answer: Not Provided**