A client who's receiving phenytoin (Dilantin) to control seizures is admitted to the health care facility for observation. The physician orders a measurement of the client's serum phenytoin level. The therapeutic serum phenytoin level is ______: a. A level below 5 mcg/mL b. 10 to 20 mcg/mL c. 25 to 35 mcg/mL d. 40 to 50 mcg/mL