Subconjunctival Hemorrhage: This condition is characterized by a sudden appearance of blood under the conjunctiva, which could be due to a minor injury, even if not recalled, or spontaneous rupture of small conjunctival blood vessels. It presents with sudden onset of redness and can be associated with minor irritation but typically lacks discharge or significant pain.

Conjunctivitis: This could be either viral or allergic in nature. Viral conjunctivitis often presents with redness, irritation, and sometimes a watery discharge. Allergic conjunctivitis is associated with itching, redness, and stringy discharge, often in the context of other allergic symptoms.
Dry Eye Syndrome: This condition can cause irritation and redness of the eye, especially if the patient has been in a dry environment or has a history of inadequate tear production.
Iritis (Anterior Uveitis): Although less common, iritis can present with sudden onset of eye pain, redness, and sensitivity to light. It is an inflammation of the iris and can be associated with systemic diseases.

Acute Angle-Closure Glaucoma: A medical emergency that presents with sudden severe eye pain, redness, blurred vision, headache, nausea, and vomiting. It is crucial to diagnose promptly to prevent permanent vision loss.
Endophthalmitis: A severe infection within the eye that can occur post-surgically or following trauma. It presents with severe eye pain, redness, decreased vision, and possibly discharge. Prompt recognition and treatment are essential to preserve vision.
Retinal Detachment: Although it typically presents with symptoms like flashes of light, floaters, and a curtain descending over the visual field, in some cases, it might initially be perceived as sudden redness and irritation if associated with significant subretinal hemorrhage.

Scleritis: A severe inflammatory disease that affects the sclera, presenting with deep, boring pain and redness. It is less common than other causes of red eye but can be associated with systemic autoimmune diseases.
Episcleritis: An inflammatory condition of the episclera, a thin layer on top of the sclera. It presents with sectoral or diffuse redness and can be associated with systemic diseases like rheumatoid arthritis.
Carotid-Cavernous Fistula: An abnormal connection between the carotid arteries and cavernous sinus, leading to increased pressure in the eye. It can present with redness, protrusion of the eye (exophthalmos), and sometimes a bruit (abnormal sound) over the eye.