Which of the following side effects is associated with long-term administration of phenytoin (Dilantin)?

Long-term administration of phenytoin is primarily associated with several side effects, including ataxia, which refers to a lack of voluntary coordination of muscle movements. This phenomenon occurs due to phenytoin's effects on the central nervous system, where it can disrupt normal neuronal firing and lead to issues with balance and coordination. Patients on long-term phenytoin therapy often show signs of cerebellar toxicity, which may manifest as ataxia.

The other side effects mentioned can also be associated with phenytoin, but they tend to be less specifically linked to the chronic use of the medication. For instance, hypotension is more commonly a concern with intravenous administration rather than long-term oral use, while osteomalacia is related to vitamin D metabolism and may occur due to long-term use but is not the most characteristic finding. Cardiac dysrhythmias can occur in cases of toxicity or rapid infusion but are not a common result of prolonged use. Thus, among the choices presented, ataxia is the most directly tied to the long-term administration of phenytoin.