**Core Concept**
Ankylosing spondylitis (AS) is a chronic inflammatory disease primarily affecting the axial skeleton, but it can also involve other systems, including the eyes. The ocular manifestations of AS are a result of chronic inflammation and can lead to significant morbidity.

**Why the Correct Answer is Right**
The most common ocular manifestation of AS is uveitis, which is an inflammation of the uvea, the middle layer of the eye. Uveitis in AS is typically bilateral and can occur at any age, with a peak incidence in the second to fourth decades of life. The pathophysiology of uveitis in AS is thought to be related to the systemic inflammation and immune dysregulation that characterizes the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while AS can cause various extra-articular manifestations, including skin and eye problems, the most common ocular manifestation is uveitis, not conjunctivitis.
**Option B:** This option is incorrect because while AS can cause a range of systemic symptoms, including fatigue and weight loss, the ocular manifestation of AS is specifically uveitis, not a systemic symptom.
**Option C:** This option is incorrect because while AS can cause inflammation in various parts of the body, the ocular manifestation of AS is specifically uveitis, not retinal detachment.
**Option D:** This option is incorrect because while AS can cause various eye problems, the most common ocular manifestation of AS is uveitis, not optic neuritis.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that uveitis in AS can be asymptomatic, and patients may present with visual disturbances or eye pain. A high index of suspicion is necessary for early diagnosis and treatment to prevent long-term visual impairment.

**Correct Answer: A. Uveitis**