What is one clinical sign of allergic conjunctivitis?

Blepharospasm is a clinical sign commonly associated with allergic conjunctivitis due to the irritation and discomfort caused by inflammation of the conjunctiva. When an animal suffers from allergic conjunctivitis, the eyes become itchy, red, and inflamed, prompting the animal to squint or tightly shut its eyelids in response to the discomfort. This involuntary spasm of the eyelids is the body's natural reflex to protect the eyes from further irritation and reduce exposure to allergens.

The other choices—pupil dilation, corneal opacity, and retinal detachment—are not characteristic signs of allergic conjunctivitis. Pupil dilation could indicate different ocular or systemic issues, corneal opacity typically results from conditions affecting the cornea rather than conjunctival inflammation, and retinal detachment involves a separation of the retina from the underlying tissue, which is unrelated to allergic conjunctivitis. Hence, blepharospasm serves as a clear and direct indicator of the condition, reflecting the underlying inflammatory response in the conjunctiva.