Corneal Abrasion and Foreign Body
Corneal Ulcer
Foreign Body
Trauma
Endophthalmitis following prior surgery, with hypopyon or pus in anterior chamber
Periorbital cellulitis/orbital cellulitis
Acute Glaucoma: corneal clouding
Scleritis
Uveitis (recurrent in AS): keratic precipitates
photophobia, perilimbal injection
Hyphema/Hypopyon
Herpes Zoster Ophthalmicus (look for concurrent skin findings)
Thyroid disease
photophobia, perilimbal injection
Red eye 
Bilateral red eyes
Ocular palsy
Retinal hemorrhage
Subconjunctival hemorrhage
Cataracts
Papilledema
Retinal detachment