Which of the following is considered an anticonvulsant?

Phenytoin is recognized as an anticonvulsant medication primarily used for the treatment of epilepsy. It stabilizes the electrical activity in the brain by inhibiting the spread of seizure activity. Phenytoin works by modulating sodium channels and is particularly effective in preventing seizures from occurring after an initial episode, such as in status epilepticus.

In contrast, heparin is an anticoagulant that prevents blood clots, nifedipine is a calcium channel blocker used to treat high blood pressure and angina, and hydrochlorothiazide is a diuretic used to manage hypertension and fluid retention. None of these options are classified as anticonvulsants, which highlights the specific role that phenytoin plays in neurologic management.