Acute Angle-Closure Glaucoma: This condition is characterized by sudden onset of severe eye pain, often accompanied by blurred vision, headache, nausea, and vomiting. It's a medical emergency requiring prompt treatment to prevent permanent vision loss. The young age of the patient might suggest other causes, but the acute presentation fits this diagnosis.

Conjunctivitis: This is an inflammation or infection of the outer membrane of the eyeball and the inner eyelid. It can cause pain, redness, and discharge. Given its commonality, it's a plausible diagnosis for unilateral eye pain.
Dry Eye Syndrome: Although more common in older populations or those with certain autoimmune diseases, dry eye can cause significant discomfort and pain, especially if severe.
Uveitis: Inflammation of the uvea, which includes the iris, ciliary body, and choroid. It can cause eye pain, sensitivity to light, and vision problems.
Corneal Abrasion or Ulcer: Damage to the cornea can cause significant pain, redness, and sensitivity to light.

Acute Angle-Closure Glaucoma (already mentioned): Due to its potential for irreversible vision loss, it's crucial not to miss this diagnosis.
Endophthalmitis: A severe inflammation of the interior of the eye, usually caused by infection. It's a medical emergency that requires immediate attention.
Retinal Detachment: A condition where the retina separates from the back of the eye, causing sudden vision loss and potentially severe eye pain.
Optic Neuritis: Inflammation of the optic nerve, which can cause pain with eye movement and vision loss.

Scleritis: A severe inflammatory disease that affects the sclera (the white part of the eye). It's less common than other causes of eye pain but can be very painful.
Orbital Cellulitis: An infection of the soft tissues surrounding the eye, which can cause pain, swelling, and vision changes.
Tolosa-Hunt Syndrome: A rare disorder characterized by headache, pain around the eye, and weakness of extraocular muscles due to inflammation of the cavernous sinus or superior orbital fissure.
Ocular Migraine: While migraines are common, ocular migraines causing significant unilateral eye pain without headache are less frequently encountered.