The most likely diagnosis associated with this patient's scleritis is granulomatosis with polyangiitis (formerly known as Wegener granulomatosis). Scleritis is inflammation of the fibrous layer of the eye underlying the conjunctiva and episclera. Scleritis can be caused by autoimmune diseases such as rheumatoid arthritis, relapsing polychondritis, inflammatory bowel disease, or vasculitis. This patient has evidence of systemic inflammation on laboratory testing, hypertension, and an abnormal urinalysis that could represent glomerulonephritis but no clinical evidence of rheumatoid arthritis, chondritis, or inflammatory bowel disease. Vasculitis, especially ANCA-associated vasculitis, should always be considered in the differential diagnosis of scleritis because delay in diagnosis can result in permanent loss of vision. In this case, the sinus symptoms, abnormal urinalysis suggestive of glomerulonephritis, and scleritis are suggestive of granulomatosis with polyangiitis.

Ankylosing spondylitis is characterized by inflammatory back pain that usually presents during the second to third decade of life. The type of ocular inflammation most commonly associated with this entity is anterior uveitis, not scleritis. Uveitis (inflammation of the uvea) commonly presents as a red eye with pain, photophobia, and blurred vision. Anterior uveitis is characterized by circumferential redness (ciliary flush) at the corneal limbus (junction of the cornea and sclera). Furthermore, the duration of this patient's back pain (which began after age 45 years) and evidence of osteoarthritis in her fingers suggest degenerative, rather than inflammatory, disease of the spine.

Behçet syndrome is characterized by recurrent oral and/or genital ulcers, eye and skin involvement, and pathergy. This patient has no clinical symptoms to suggest this diagnosis, and it is more commonly associated with uveitis rather than scleritis.

Sarcoidosis is a multisystem disease characterized by granulomas that form in tissues and most commonly affects the lungs. Sarcoidosis is more likely to cause uveitis than scleritis, and this patient has no other clinical symptoms such as respiratory complaints or an abnormal chest radiograph to suggest this diagnosis.