**Core Concept**
Theophylline is a methylxanthine derivative used in the treatment of asthma and chronic obstructive pulmonary disease (COPD). Its clearance is influenced by various factors, including pharmacokinetic interactions.

**Why the Correct Answer is Right**
Theophylline is primarily metabolized by the cytochrome P450 1A2 (CYP1A2) enzyme in the liver. Certain drugs can induce or inhibit this enzyme, thereby affecting theophylline levels. Cimetidine is a well-known inhibitor of CYP1A2, which can lead to increased levels of theophylline. This is because cimetidine competes with theophylline for the active site of the enzyme, reducing its metabolism and resulting in higher concentrations of the drug.

**Why Each Wrong Option is Incorrect**
* **Option A:** Ciprofloxacin is a CYP1A2 inducer, which would decrease theophylline levels, not increase them.
* **Option B:** Ethanol is metabolized by CYP2E1, which does not significantly interact with theophylline metabolism.
* **Option D:** Rifampicin is a potent inducer of CYP1A2, which would decrease theophylline levels.

**Clinical Pearl / High-Yield Fact**
When prescribing theophylline, it is essential to consider potential drug interactions, particularly with CYP1A2 inhibitors like cimetidine, to avoid toxicity.

**Correct Answer:** C. Cimetidine