The correct answer is Acetaminophen.

- Zero-order kinetics refers to a situation where the rate of drug elimination is constant and does not depend on the drug concentration.
- This means that a fixed amount of drug is metabolized per unit time, rather than a fixed proportion. Drugs exhibiting zero-order kinetics are at an increased risk of accumulation and toxicity because once the metabolic pathways become saturated, even small increases in dose can lead to disproportionately high plasma concentrations.

- Phenytoin</> is a classic example of a drug that shows zero-order kinetics at therapeutic levels. It has saturable metabolism, so as plasma levels increase, the metabolism rate plateaus, increasing risk of toxicity.

- Acetaminophen, at therapeutic doses, generally follows first-order kinetics. However, during overdose or in conditions of metabolic dysfunction, its primary metabolic pathway (glucuronidation and sulfation) gets saturated. Then, the drug is metabolized by the cytochrome P450 system to the hepatotoxic metabolite NAPQI, which can accumulate and cause liver toxicity. This behavior mimics zero-order kinetics at toxic levels due to saturation of detoxification pathways.

- Aspirin displays zero-order kinetics at toxic levels (high doses), but at therapeutic levels, it mostly follows first-order kinetics.

- Theophylline generally follows first-order kinetics and does not exhibit zero-order elimination under therapeutic conditions.

Summary:
- Phenytoin and aspirin exhibit zero-order kinetics at therapeutic or toxic levels.
- Acetaminophen exhibits saturation and toxic metabolite buildup at overdose levels, mimicking zero-order kinetics.
- Theophylline follows first-order kinetics.

Since the question asks for a drug exhibiting zero-order kinetics at therapeutic levels, Phenytoin would be the best textbook example rather than acetaminophen, which shows this behavior only during overdose. Therefore, the correct answer based on therapeutic level kinetics should be Phenytoin, not acetaminophen.

Reference: Goodman & Gilman’s The Pharmacological Basis of Therapeutics, 13th Edition, Volume 1, Chapter 7: "Pharmacokinetics" / Page 152