Final answer:
The most common cause of trichiasis is chronic conjunctivitis caused by Chlamydia trachomatis, which leads to scarring and inward turning of the eyelashes. This can result in blindness if not treated.
Step-by-step explanation:
The most common cause of trichiasis, which is the inward turning of eyelashes towards the eyeball, is chronic conjunctivitis caused by the bacterium Chlamydia trachomatis. Infection by C. trachomatis leads to the formation of necrotic follicles and scarring in the upper eyelid, which causes the eyelashes to turn inward. This mechanical abrasion of the cornea can eventually lead to blindness if not corrected by surgery.