**Core Concept**
Seizures in a preterm infant can be caused by various factors, including hypoxic-ischemic encephalopathy, intraventricular hemorrhage, or metabolic disturbances. The choice of antiepileptic medication is critical in this context, as some medications may have adverse effects on the developing brain.

**Why the Correct Answer is Right**
Phenytoin is the antiepileptic medication of choice for seizures in a preterm infant. This is because phenytoin has a relatively narrow therapeutic index and can be titrated carefully to achieve therapeutic levels. Additionally, phenytoin is less likely to cause sedation or respiratory depression, which can be problematic in a preterm infant. Phenytoin works by blocking voltage-gated sodium channels, which reduces the frequency of action potentials and subsequent neuronal excitability.

**Why Each Wrong Option is Incorrect**
**Option A:** Phenytoin is not the first-line treatment for seizures in a preterm infant, and its use may be associated with adverse effects such as gingival hyperplasia and hirsutism.
**Option B:** Phenytoin is not the preferred antiepileptic medication in this context, as it may not be as effective as other options and has a narrower therapeutic index.
**Option C:** Phenytoin is not the correct answer, and its use may be associated with adverse effects such as sedation and respiratory depression.

**Clinical Pearl / High-Yield Fact**
In a preterm infant with seizures, it is essential to consider the underlying cause and choose an antiepileptic medication that is effective and has a favorable safety profile. Phenytoin may be a good option in this context, but its use should be carefully monitored and titrated to achieve therapeutic levels.

**Correct Answer: C. Phenobarbital**