**Core Concept**
In acute anterior uveitis, the pupil is often affected due to inflammation of the iris and ciliary body. This condition leads to a complex interplay between the iris sphincter and dilator muscles, resulting in a characteristic change in pupil size and reactivity.

**Why the Correct Answer is Right**
The pupil in acute anterior uveitis is typically constricted and sluggish reacting due to the release of various mediators and cytokines during the inflammatory process. These substances cause the iris sphincter muscle to contract, leading to miosis. Additionally, the inflammation can cause the iris to become stuck in this constricted state, resulting in a sluggish pupillary response to light.

**Why Each Wrong Option is Incorrect**
* **Option A:** Semi-dilated pupils are often seen in conditions such as Horner's syndrome, where there is a disruption of the sympathetic innervation to the eye.
* **Option B:** Large and fixed pupils are characteristic of a midbrain lesion or a third cranial nerve palsy, where there is a disruption of the parasympathetic innervation to the iris sphincter muscle.
* **Option C:** Irregular and constricted pupils are often seen in conditions such as Adie's tonic pupil or in cases of iris trauma, where there is damage to the iris sphincter muscle.

**Clinical Pearl / High-Yield Fact**
In cases of acute anterior uveitis, it's essential to perform a thorough examination of the pupil, including assessment of its size, shape, and reactivity to light. A sluggish pupillary response can be a key indicator of uveitis, and it's essential to differentiate this from other conditions that may cause a similar presentation.

**Correct Answer:** D. Constricted & sluggish reacting.