**Core Concept**
Acute anterior uveitis is an inflammatory condition of the anterior segment of the eye, characterized by the involvement of the iris and ciliary body. It can present with a range of symptoms, including pain, redness, and sensitivity to light, as well as changes in the pupil's response.

**Why the Correct Answer is Right**
In acute anterior uveitis, the pupil is typically **small and irregular** due to the inflammation and spasm of the iris sphincter muscle. This is known as a **miosis** or **pupillary spasm**. The inflammation can cause the iris to become stuck to the lens (posterior synechiae), leading to a decrease in the size of the pupil. Additionally, the pain and inflammation can cause the iris sphincter muscle to contract, resulting in a smaller pupil.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because in acute anterior uveitis, the pupil is not typically dilated. A dilated pupil would suggest a different condition, such as a third cranial nerve palsy or an ischemic optic neuropathy.
**Option B:** This option is incorrect because a normal-sized pupil would not be consistent with the inflammation and spasm of the iris sphincter muscle seen in acute anterior uveitis.
**Option C:** This option is incorrect because a fixed and dilated pupil would suggest a more severe condition, such as a sympathetic ophthalmia or a traumatic injury to the eye.

**Clinical Pearl / High-Yield Fact**
A key feature of acute anterior uveitis is the presence of **keratic precipitates** (KPs) on the corneal endothelium, which are small white or grayish deposits of inflammatory cells. The presence of KPs is a diagnostic clue for acute anterior uveitis.

**Correct Answer:** C. A small and irregular pupil is a characteristic finding in acute anterior uveitis, making option C the correct answer.