Ethosuximide in Typical Absence

An 8-year-old girl with childhood absence epilepsy (CAE) presents for follow-up EEG one month after starting ethosuximide monotherapy. She had been experiencing 20–30 absence seizures daily before treatment. Her baseline EEG showed classic 3 Hz generalized spike-and-wave discharges provoked by hyperventilation. On current EEG, hyperventilation and photic stimulation fail to elicit any spike-and-wave activity, and the awake tracing now shows the pattern marked **B** — a well-organized posterior dominant alpha rhythm at 9 Hz that attenuates appropriately with eye opening, with normal sleep architecture. Which of the following best explains the mechanism by which ethosuximide produces this electrographic normalization shown at **B**?