**Core Concept:** Acute iridocyclitis is an inflammation of the iris and ciliary body, which can be caused by various factors such as infections, autoimmune disorders, or drug reactions. These inflammatory processes can lead to the disruption of the blood-aqueous barrier, resulting in symptoms and signs characteristic of iridocyclitis.

**Why the Correct Answer is Right:** The pathognomonic sign of acute iridocyclitis is the presence of hypopyon, which is a key feature of this condition. Hypopyon is defined as the accumulation of inflammatory cells (predominantly neutrophils) in the anterior chamber of the eye, leading to a dense, opaque, and elevated layer above the cornea. This accumulation of inflammatory cells results from the breakdown of the blood-aqueous barrier, allowing immune cells to enter the anterior chamber and cause the characteristic appearance. The presence of hypopyon helps differentiate iridocyclitis from other anterior uveitis conditions and facilitates the appropriate management and treatment.

**Why Each Wrong Option is Incorrect:**

A. **Iris distortion** is not a specific sign of iridocyclitis, as it can be seen in various other anterior uveitis conditions as well.
B. **Hypotony** (low intraocular pressure) is a rare and nonspecific finding in iridocyclitis and can be seen in other ocular conditions, making it an unsuitable answer.
C. **Anterior uveitis** is a broad term that includes iridocyclitis, but the presence of hypopyon is specific to iridocyclitis.
D. **Hyphema** refers to the presence of blood within the anterior chamber, which is different from the accumulation of inflammatory cells seen in hypopyon. Hyphema is less specific than hypopyon for diagnosing iridocyclitis, as it can be seen in other ocular conditions involving bleeding.

**Clinical Pearl:** The pathognomonic sign of acute iridocyclitis is the presence of hypopyon, which is highly specific for this condition. This feature helps in distinguishing iridocyclitis from other anterior uveitis conditions and guides appropriate management decisions.

**Correct Answer:** C. **Anterior uveitis**.

**Why the Correct Answer is Right:** The correct answer is "Anterior uveitis" because the question is asking for a specific feature of iridocyclitis, which is a subtype of anterior uveitis. Hypopyon is a characteristic feature of iridocyclitis and distinguishes it from other anterior uveitis conditions like panuveitis or posterior uveitis. The presence of hypopyon in the anterior chamber of the eye is a key diagnostic criterion for iridocyclitis.