Phenytoin is primarily used to treat which of the following conditions?

Phenytoin is an anticonvulsant medication that is primarily used to manage seizure disorders, particularly tonic-clonic seizures and focal seizures. It works by stabilizing the neuronal membranes and reducing the excessive electrical activity that leads to seizures. Phenytoin is well-established in the treatment of epilepsy and is often used when other medications are ineffective or when a specific seizure type warrants its use.

This drug does not have a role in treating atrial fibrillation, anxiety disorders, or acute pain management. Atrial fibrillation requires antiarrhythmic agents or anticoagulants; anxiety disorders are typically treated with anxiolytics or antidepressants; and acute pain management often involves analgesics or nonsteroidal anti-inflammatory drugs (NSAIDs). Therefore, Phenytoin's primary indication as an anticonvulsant solidifies its role in the management of seizure disorders.