Which of the following symptoms is characteristic of a hordeolum?

A hordeolum, commonly known as a stye, is characterized primarily by unilateral eyelid swelling. This condition occurs when a gland along the eyelid becomes infected, leading to a localized area of redness, swelling, and pain, usually on the outer part of the eyelid. The swelling is typically noticeable and can be accompanied by tenderness in the affected area.

While changes in vision, constant tearing, and severe redness of the eye might occur in various eye conditions, they are not specifically typical for hordeolum. Changes in vision often indicate other underlying issues such as refractive errors or infections that affect the eye itself, rather than the eyelid. Constant tearing can signify other problems such as conjunctivitis or dry eye syndrome, and severe redness of the eye is more often associated with conditions like conjunctivitis or acute glaucoma. Therefore, the unilateral eyelid swelling reflects the hallmark feature of a hordeolum, making it the most appropriate symptom to identify this condition.