**Core Concept**
Toxic doses and zero-order kinetics are related to the pharmacokinetics of certain medications. Zero-order kinetics occurs when the rate of elimination of a drug is constant and independent of the drug's concentration in the body.

**Why the Correct Answer is Right**
Phenytoin is a classic example of a medication that exhibits zero-order kinetics at toxic doses. At high concentrations, the enzyme responsible for its metabolism, cytochrome P450 (CYP2C9), becomes saturated, leading to a constant rate of elimination regardless of the plasma concentration. This results in a disproportionate increase in plasma levels and toxicity, even with small increases in dosage. This phenomenon is known as the "zero-order" or "saturation" kinetics.

**Why Each Wrong Option is Incorrect**
**Option A:** Penicillin is eliminated primarily through renal excretion and exhibits first-order kinetics, where the rate of elimination is proportional to the plasma concentration.

**Option C:** Valproate is primarily metabolized by glucuronidation and also exhibits first-order kinetics.

**Option D:** Carbamazepine is metabolized by CYP3A4 and exhibits first-order kinetics.

**Clinical Pearl / High-Yield Fact**
Zero-order kinetics can lead to a narrow therapeutic index, making it essential to monitor plasma levels and adjust dosages carefully to avoid toxicity. Remember the "Phenytoin Paradox": even small increases in dosage can lead to significant increases in plasma levels and toxicity due to saturation kinetics.

**✓ Correct Answer: B. Phenytoin**