All of the following statements about phenytoin are true except:
**Core Concept**
Phenytoin is an antiepileptic medication that belongs to the class of hydantoins. It works by inhibiting voltage-gated sodium channels in the nervous system, thereby reducing the frequency of action potentials and subsequent seizure activity.
**Why the Correct Answer is Right**
Phenytoin has a unique pharmacokinetic profile characterized by zero-order kinetics at therapeutic doses, meaning that small changes in dose can lead to disproportionately large changes in plasma concentrations. This is due to the saturation of the enzyme responsible for its metabolism, cytochrome P450 2C9. As a result, phenytoin can accumulate to toxic levels if doses are not carefully adjusted. Additionally, phenytoin is known for its nonlinear kinetics, which can lead to a narrow therapeutic index.
**Why Each Wrong Option is Incorrect**
**Option A:** This statement is not necessarily true, as phenytoin is indeed metabolized by the liver, primarily by the cytochrome P450 system, but it also undergoes glucuronidation.
**Option B:** This statement is true, as phenytoin is known to cause gingival hyperplasia, a condition characterized by overgrowth of the gums, particularly in children and adolescents.
**Option C:** This statement is true, as phenytoin can cause fetal hydantoin syndrome in the offspring of pregnant women taking the medication.
**Option D:** This statement is true, as phenytoin is associated with a range of adverse effects, including nystagmus, ataxia, and dysarthria, particularly at higher doses.
**Clinical Pearl / High-Yield Fact**
To avoid toxicity, it's essential to monitor phenytoin levels regularly, especially when initiating or adjusting therapy, and to use a loading dose to achieve therapeutic levels quickly.
**Correct Answer: D. Phenytoin is generally not associated with this particular side effect.**