**Core Concept**
Theophylline is a methylxanthine derivative used in the treatment of asthma and chronic obstructive pulmonary disease (COPD). It acts as a phosphodiesterase inhibitor, increasing cyclic AMP (cAMP) levels, and also has adenosine receptor antagonist properties.

**Why the Correct Answer is Right**
Theophylline's mechanism of action is multifaceted. As a phosphodiesterase inhibitor, it prevents the breakdown of cAMP, leading to smooth muscle relaxation in the airways. This results in bronchodilation and improved lung function. Additionally, theophylline's adenosine receptor antagonism further contributes to its bronchodilatory effects. This dual action makes theophylline an effective treatment for asthma and COPD.

**Why Each Wrong Option is Incorrect**
**Option A:** This option may be incorrect if it contradicts the known mechanisms of theophylline.
**Option B:** This option may be incorrect if it suggests theophylline has a different primary mechanism of action.
**Option C:** This option may be incorrect if it inaccurately describes the effects of theophylline.

**Clinical Pearl / High-Yield Fact**
Theophylline has a narrow therapeutic index, and its plasma concentration should be carefully monitored to avoid toxicity. Common side effects of theophylline include nausea, vomiting, and headache.

**Correct Answer:** C.