**Core Concept**
Prolonged phenytoin therapy can lead to various side effects due to its interference with cellular processes, including enzyme induction and inhibition. Phenytoin is a well-known antiepileptic medication that affects the activity of several enzymes and ion channels in the body.

**Why the Correct Answer is Right**
Phenytoin's side effects can be attributed to its ability to induce the cytochrome P450 enzyme system, leading to the increased metabolism of other drugs, and its inhibition of the sodium channel, resulting in altered neuronal excitability. Additionally, phenytoin can cause hirsutism, gingival hyperplasia, and megaloblastic anemia due to its interference with folate metabolism.

**Why Each Wrong Option is Incorrect**
**Option A:**
* Hirsutism is indeed a side effect of prolonged phenytoin therapy, making this option incorrect.

**Option B:**
* Ataxia is a well-known side effect of phenytoin, particularly at high doses, due to its effects on cerebellar function and coordination.

**Option C:**
* Hypersensitivity reactions, including a rash and fever, can occur with phenytoin, especially during the initial stages of treatment.

**Clinical Pearl / High-Yield Fact**
It is essential to monitor patients on long-term phenytoin therapy for signs of gingival hyperplasia, which can be a sign of impending cardiovascular complications.

**Correct Answer: D.**