This patient has uveitis, which is inflammation involving the middle structures of the eye (iris, ciliary body, and choroid). It usually presents with unilateral eye pain, redness, and photophobia. The characteristic finding on physical examination is circumferential redness around the border of the sclera and cornea (corneal limbus), termed ciliary flush, which represents dilated conjunctival vessels. Pupillary miosis is also often seen. Slit lamp examination commonly reveals inflammatory (“flare”) cells in the anterior chamber. Uveitis can either be idiopathic or occur as part of an underlying systemic condition, such as autoimmune disorders, arthritides associated with HLA-B27 antigen, infection (syphilis, tuberculosis, herpes simplex virus), malignancy, and sarcoidosis. This patient has symptoms suggestive of ankylosing spondylitis, including fatigue and low back pain that awakens her from sleep and improves with activity and NSAID therapy. Ankylosing spondylitis is a systemic condition that may be associated with uveitis.

Corneal ulcers are caused by trauma, contact lens wear, herpes simplex virus infection, bacterial infection, and connective tissue disorders (ankylosing spondylitis). Although corneal ulcers can occur in the setting of ankylosing spondylitis, this patient's ciliary flush is more characteristic of uveitis.

Patients with episcleritis typically present with redness, irritation, and tearing but not significant ocular pain. Additionally, the redness seen in episcleritis is usually more widespread rather than limited to the perilimbic region of the involved eye, making this an unlikely diagnosis in this patient.

Scleritis can be associated with autoimmune disorders; however, it typically presents bilaterally, involves redness across the entire sclera, and does not present with ciliary flush.