**Core Concept**
Acute iridocyclitis is a form of uveitis that involves inflammation of the iris and ciliary body. The pathognomonic sign refers to a specific clinical feature that is uniquely characteristic of this condition.

**Why the Correct Answer is Right**
The correct answer is **halo around lights**. This phenomenon occurs due to the accumulation of inflammatory cells and exudates in the anterior chamber of the eye, which scatter light and create a characteristic halo around lights. This symptom is particularly pronounced in low light conditions and is often described by patients as seeing a "halo" or "glow" around objects.

**Why Each Wrong Option is Incorrect**
**Option A:** Keratic precipitates (KPs) are a sign of uveitis, but they are not pathognomonic for acute iridocyclitis. While KPs can be present in this condition, they can also be seen in other forms of uveitis.

**Option B:** Eye pain is a common symptom of uveitis, but it is not a specific sign of acute iridocyclitis. Eye pain can be present in a variety of ocular conditions and is not unique to this condition.

**Option C:** The presence of a flare in the anterior chamber is a sign of uveitis, but it is not pathognomonic for acute iridocyclitis. Flare can be seen in other forms of uveitis, including posterior uveitis.

**Clinical Pearl / High-Yield Fact**
In addition to a halo around lights, patients with acute iridocyclitis may also experience eye pain, redness, and photophobia. It's essential to conduct a thorough ophthalmic examination, including slit-lamp biomicroscopy, to diagnose and manage this condition effectively.

**Correct Answer: A. Halo around lights**