Q. A patient with longstanding, chronic epilepsy is stable on an antiepileptic drug (Drug A) suddenly develops a drug rash with the addition of a second antiepileptic drug (Drug B). You suspect this is due to an increase in the blood level of Drug A with the addition of Drug B to the regimen. In addition to taking measures to treat and prevent worsening of the rash, you decide to send off a blood level for which medication?