Correct Answer: A

Rationale: Phenytoin stabilizes neuronal membranes and prevents seizure propagation by prolonging the inactivation state of voltage-gated sodium channels, thereby reducing repetitive neuronal firing. This mechanism selectively targets hyperactive neurons without depressing overall CNS activity. The achievement of seizure control upon reaching therapeutic plasma levels indicates optimal sodium channel modulation. Phenytoin does not directly enhance GABA or inhibit NMDA receptors, differentiating it from benzodiazepines or newer anticonvulsants. Its mechanism underlies its efficacy in tonic-clonic and partial seizures.

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