Which feature best distinguishes phenytoin from valproic acid in terms of mechanism of action and clinical use?

Explanation

## Mechanism of Action Comparison ### Phenytoin **Key Point:** Phenytoin acts primarily by **blocking voltage-gated sodium channels**, stabilizing neuronal membranes and raising the seizure threshold. - Mechanism: Prolongs the inactivated state of Na^+^ channels - Effect: Prevents repetitive neuronal firing - Spectrum: Partial seizures and generalized tonic-clonic seizures (not absence) ### Valproic Acid **Key Point:** Valproic acid has a **dual mechanism**: increases GABA synthesis AND blocks T-type calcium channels. - Primary: Increases GABA levels (inhibitory neurotransmitter) - Secondary: Blocks calcium channels - Spectrum: Broad-spectrum — effective for absence, generalized tonic-clonic, myoclonic, and partial seizures ## Comparison Table | Feature | Phenytoin | Valproic Acid | | --- | --- | --- | | **Primary MOA** | Na^+^ channel blockade | GABA enhancement + Ca^2+^ channel blockade | | **Spectrum** | Partial & GTCS (NOT absence) | Broad-spectrum (all seizure types) | | **Enzyme Effect** | Inducer (CYP3A4, 2C9) | Inhibitor (CYP2C9, 3A4) | | **Teratogenicity** | Fetal hydantoin syndrome | Highest teratogenic risk (NTDs, developmental delay) | ## High-Yield Distinction **High-Yield:** The **sodium channel blockade** of phenytoin vs. **GABA enhancement** of valproate is the fundamental mechanistic difference that explains their clinical profiles: - Phenytoin: membrane stabilization → good for partial seizures - Valproate: GABAergic enhancement → broad-spectrum efficacy **Clinical Pearl:** Valproic acid is the **drug of choice for absence seizures** (where phenytoin is ineffective), precisely because absence seizures respond to GABA enhancement, not sodium channel blockade. **Warning:** Do NOT confuse phenytoin's enzyme induction with valproate's enzyme inhibition — this affects drug interactions profoundly and is a common exam trap.