A patient who is being treated for temporal lobe epilepsy (complex partial seizures) and who is having recurrent seizures on his chronic regimen of carbamazepine is given phenobarbital as a second drug. However, the seizures increase in frequency. What is the probable reason for the apparent deleterious effect of adding phenobarbital -
Intracerebral bleeding from worsening bone marrow supression
Decreased carbamazepine level
Decreased stability of CNS neuronal membranes
Hypokalemia