## Core Concept
Diphenylhydantoin, also known as phenytoin, is an antiepileptic medication that can cause a range of side effects due to its effects on the central nervous system (CNS) and other bodily systems. The question requires the test-taker to identify which option is NOT a side effect of diphenylhydantoin.

## Why the Correct Answer is Right
The correct answer, , is associated with a decrease in side effects related to gingival hyperplasia and hirsutism which are commonly seen with diphenylhydantoin. Diphenylhydantoin can cause gingival hyperplasia, a condition characterized by overgrowth of gum tissue, and hirsutism, which is excessive hair growth.

## Why Each Wrong Option is Incorrect
* **Option A:** - Gingival hyperplasia is indeed a known side effect of diphenylhydantoin.
* **Option B:** - Ataxia and nystagmus are CNS-related side effects of diphenylhydantoin due to its effect on the cerebellum.
* **Option C:** - Hirsutism is another recognized side effect.

## Clinical Pearl / High-Yield Fact
A key point to remember is that diphenylhydantoin can cause a range of side effects, but it is particularly known for its effects on the gums (gingival hyperplasia) and its potential to cause neurological side effects like ataxia and nystagmus at toxic levels.

## Correct Answer: D.