Ms. Rutz was admitted with a new diagnosis of seizure disorder and has finally been controlled on a combination of antiepileptic medications. This morning's laboratory results report that Ms. Rutz' phenytoin level is 14 mg/L. The nurse practitioner recognizes that this represents a good therapeutic range because phenytoin should fall between 10 and 20 mg/dL. The range between 10 and 20 mg/L represents the:
A. Average daily dose
B. Therapeutic window
C. Peak and trough
D. First order metabolism