**Core Concept**
Dilantin sodium, also known as phenytoin, is an antiepileptic medication that works by blocking voltage-gated sodium channels in the brain, thereby reducing the frequency of seizures. Its side effects are primarily related to its effects on the nervous system, cardiovascular system, and liver metabolism.

**Why the Correct Answer is Right**
The correct answer is **D. Gum Hypertrophy**. Gum hypertrophy, also known as gingival overgrowth, is a well-known side effect of phenytoin, caused by its effect on the fibroblasts in the gingiva, leading to increased collagen deposition and subsequent hypertrophy of the gingival tissue. This is due to the inhibition of the enzyme phosphodiesterase by phenytoin, which leads to increased levels of cyclic AMP (cAMP) in the gingival fibroblasts, promoting collagen synthesis.

**Why Each Wrong Option is Incorrect**
**Option A:** Although phenytoin can cause ataxia, this is not as specific or as well-known as gum hypertrophy.
**Option B:** Phenytoin can cause hirsutism, but this is not as common as gum hypertrophy.
**Option C:** Phenytoin can cause nystagmus, but this is not as closely associated with the medication as gum hypertrophy.

**Clinical Pearl / High-Yield Fact**
Gum hypertrophy is a classic side effect of phenytoin, and it's essential to monitor patients on this medication for signs of gingival overgrowth, which can lead to poor oral hygiene and increased risk of dental problems.

**Correct Answer:** D. Gum Hypertrophy