**Core Concept**
Theophylline is a methylxanthine derivative used in asthma and COPD management. Its therapeutic blood level is tightly regulated due to narrow therapeutic index and risk of toxicity, with efficacy and safety dependent on plasma concentration.

**Why the Correct Answer is Right**
Theophylline has a narrow therapeutic window. The therapeutic range for effective bronchodilation without significant toxicity is **20–50 µg/L**. At levels above 50 µg/L, adverse effects such as arrhythmias, seizures, and tachycardia occur. This range is established through clinical studies and pharmacokinetic data, with 20 µg/L being the minimum effective dose and 50 µg/L the upper limit before toxicity.

**Why Each Wrong Option is Incorrect**
Option A: 0–5 µg/L – This range is too low and falls below the effective therapeutic level; patients would not experience bronchodilation.
Option B: 10–50 µg/L – While 50 µg/L is correct, the lower limit of 10 µg/L is subtherapeutic; theophylline is ineffective at such low levels.
Option C: 15–50 µg/L – The lower limit of 15 µg/L is still suboptimal; therapeutic efficacy begins at 20 µg/L.

**Clinical Pearl / High-Yield Fact**
Always monitor theophylline levels closely; even small fluctuations can lead to toxicity. Avoid combining with other drugs that affect metabolism (e.g., cimetidine, erythromycin), as they can increase plasma levels and risk of adverse effects.

✓ Correct Answer: D. 20–50 µg/L