A 67-year-old lawyer has a triangular thickening of the conjunctiva. What is the most likely diagnosis?

The presence of a triangular thickening of the conjunctiva in this case suggests a diagnosis of pterygium. Pterygium is characterized by the abnormal growth of conjunctival tissue that extends onto the cornea, typically in a triangular shape. It is often associated with exposure to UV light, leading to an increase in incidence among individuals who spend a lot of time outdoors.

This condition is typically seen in older adults and can sometimes cause discomfort, visual disturbances, or cosmetic concerns. The trajectory of the growth, which usually progresses from the nasal side of the eye outward, aligns with the description provided in the question.

Pinguecula, although it may also present as a thickening on the conjunctiva, is usually a yellowish, raised patch that does not extend onto the cornea in a triangular shape. Cataract refers to opacification of the lens of the eye, which would not appear as a thickening of the conjunctiva, and a chalazion is a cyst in the eyelid caused by blocked oil glands, distinctly different from conjunctival thickening. Each of these conditions has unique presentations, with pterygium being the most fitting diagnosis given the symptom described.