**Core Concept**
Theophylline is a methylxanthine derivative used in the management of bronchial asthma, and its plasma concentration needs to be monitored due to the narrow therapeutic index and potential for toxicity.

**Why the Correct Answer is Right**
Theophylline works by inhibiting phosphodiesterase, an enzyme that breaks down cyclic AMP (cAMP), thereby increasing intracellular cAMP levels and leading to smooth muscle relaxation in the airways. However, the plasma concentration of theophylline must be carefully monitored to avoid toxicity, as it can cause severe side effects such as seizures, arrhythmias, and even death at high levels. The therapeutic index of theophylline is narrow, meaning that the difference between the effective dose and the toxic dose is small.

**Why Each Wrong Option is Incorrect**
**Option B:** Cromoglycate is a mast cell stabilizer that works by preventing the release of histamine and other mediators from mast cells, but it does not require monitoring of plasma concentrations. Cromoglycate is generally safe and well-tolerated.
**Option C:** Salmeterol is a long-acting beta-2 agonist that works by stimulating beta-2 receptors in the airways, causing bronchodilation. Salmeterol does not require monitoring of plasma concentrations, and its side effects are generally mild.
**Option D:** Terbutaline is a short-acting beta-2 agonist that works by stimulating beta-2 receptors in the airways, causing bronchodilation. Terbutaline does not require monitoring of plasma concentrations, and its side effects are generally mild.

**Clinical Pearl / High-Yield Fact**
Theophylline is often used in combination with other medications, such as beta-agonists and corticosteroids, to manage bronchial asthma. However, the use of theophylline is limited by its narrow therapeutic index and potential for toxicity, making it essential to monitor plasma concentrations carefully.

**✓ Correct Answer: A. Theophylline**