**Core Concept**
Zero-order kinetics is a type of pharmacokinetic behavior where the rate of drug elimination is constant and independent of the drug's concentration. This means that the rate of elimination remains the same regardless of the plasma concentration of the drug, leading to a decrease in the drug's elimination rate as its concentration decreases.

**Why the Correct Answer is Right**
Theophylline exhibits zero-order kinetics at high concentrations due to its metabolism by cytochrome P450 enzymes. At low concentrations, theophylline is metabolized by first-order kinetics. However, at high concentrations, the enzymes responsible for its metabolism become saturated, leading to a decrease in the rate of metabolism and a subsequent increase in plasma concentration. This results in a plateau in plasma concentration, characteristic of zero-order kinetics.

**Why Each Wrong Option is Incorrect**
**Option A:** Digoxin follows first-order kinetics and is primarily eliminated by the kidneys. Its elimination is directly proportional to its plasma concentration.
**Option C:** Phenobarbitane also follows first-order kinetics and is primarily eliminated by the liver through glucuronidation. Its elimination is directly proportional to its plasma concentration.
**Option D:** Etomidate follows first-order kinetics and is primarily eliminated by the liver through glucuronidation. Its elimination is directly proportional to its plasma concentration.

**Clinical Pearl / High-Yield Fact**
Zero-order kinetics can lead to accumulation of toxic levels of a drug, especially in patients with liver disease or those taking multiple medications that interact with the same metabolic pathway.

✓ Correct Answer: B. Theophylline