This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
A patient who is being treated for temporal lobe epilepsy (complex paial 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: 
Select and explain. 
A. Intracerebral bleeding from worsening bone marrow suppression, B. Decreased carbamazepine level, C. Decreased stability of CNS neuronal membranes, D. Hypokalemia
B: Decreased carbamazepine level
Answer: B: Decreased carbamazepine level 
Explanation: Answer is B (Decreased Carbamazepine levels): Phenobarbitone is a microsomal enzyme inducer so administration of phenobarbitone induces the metabolism of carbamazepine and reduces its level in blood.