**Core Concept**
Phenytoin is an antiepileptic medication that belongs to the class of hydantoins. Its therapeutic level is crucial for maintaining seizure control while minimizing side effects.

**Why the Correct Answer is Right**
Phenytoin's therapeutic level is typically considered to be between 10-20 mcg/mL. This narrow therapeutic index is due to the nonlinear pharmacokinetics of the drug, where small increments in dose lead to disproportionately large increases in plasma concentration. The enzyme responsible for phenytoin metabolism is CYP2C19, which is also involved in the metabolism of other drugs like warfarin and omeprazole.

**Why Each Wrong Option is Incorrect**
**Option A:** Values below 5 mcg/mL are generally considered subtherapeutic, increasing the risk of seizures.
**Option B:** Although levels above 30 mcg/mL are associated with increased toxicity, this option is too high to be considered therapeutic.
**Option C:** This value falls within the therapeutic range but is not the most commonly accepted range for phenytoin.

**Clinical Pearl / High-Yield Fact**
Phenytoin's nonlinear pharmacokinetics can lead to "dose-dependent" changes in plasma concentration, making it essential to monitor levels closely, especially when initiating or adjusting therapy.

**Correct Answer: C. 15-20 mcg/mL**