A 23-year-old man presents to the clinic for evaluation of new symptoms of low back pain, stiffness, and left eye discomfort. Sunlight also bothers his eyes. The back pain is worse at night and described as a dull ache in the back and buttock area. On physical examination, there is paravertebral muscle, iliac crest, and ischial tuberosity tenderness on palpation. Active range of motion reveals limited forward flexion of the lumbar spine. His eye is also inflamed and the pupil is constricted. Lumbar and pelvic x-rays show sacroiliitis. Which of the following is the most likely diagnosis for his eye symptoms (it is the most common extra-articular manifestation of this condition)?

**Core Concept**
The patient's symptoms, including eye discomfort, pupil constriction, and sacroiliitis, suggest an inflammatory condition affecting both the axial skeleton and the eyes. This is a classic presentation of a systemic inflammatory disorder with articular and extra-articular manifestations.

**Why the Correct Answer is Right**
The patient's eye symptoms, particularly the constricted pupil and inflammation, are characteristic of uveitis, an inflammatory condition affecting the uvea, the middle layer of the eye. The most common extra-articular manifestation of ankylosing spondylitis (AS), a type of seronegative spondyloarthropathy, is anterior uveitis (also known as iridocyclitis). This condition is thought to result from the abnormal immune response of the HLA-B27 gene, which is strongly associated with ankylosing spondylitis.

**Why Each Wrong Option is Incorrect**
**Option A:** While conjunctivitis is an inflammatory condition affecting the eyes, it is characterized by redness, itching, and discharge, rather than pupil constriction and inflammation. Conjunctivitis is not the most common extra-articular manifestation of ankylosing spondylitis.

**Option B:** Optic neuritis is a condition affecting the optic nerve, leading to vision loss and pain with eye movement. While it is an inflammatory condition, it is not the most common extra-articular manifestation of ankylosing spondylitis.

**Option C:** Keratitis is an inflammatory condition affecting the cornea, leading to pain, redness, and visual disturbances. While it is an inflammatory condition affecting the eyes, it is not the most common extra-articular manifestation of ankylosing spondylitis.

**Clinical Pearl / High-Yield Fact**
Ankylosing spondylitis is a type of seronegative spondyloarthropathy, which also includes conditions such as psoriatic arthritis, reactive arthritis, and enteropathic arthritis. These conditions share common genetic and environmental risk factors, including the presence of the HLA-B27 gene.

**Correct Answer:** C.

✓ Correct Answer: B. acute anterior uveitis