An 85 year old patient experiences sudden loss of vision in the right eye, described as "a curtain came down". What is the most likely diagnosis?

The description of a sudden loss of vision characterized by the sensation of "a curtain coming down" strongly supports the diagnosis of retinal detachment. This visual phenomenon occurs due to the separation of the retinal layer from the underlying tissue, leading to a sudden and often dramatic loss of vision. Patients frequently describe it as a shadow or curtain obscuring part of their vision because the detached retina no longer functions properly to detect light, resulting in immediate visual implications.

In contrast, while vitreous hemorrhage can cause sudden loss of vision, it is typically associated with visual floaters or flashes and doesn't usually create the "curtain" sensation. An optic nerve lesion may lead to vision loss as well, but it typically presents with other symptoms such as changes in color vision or a decrease in visual field. Central retinal artery occlusion can cause sudden vision loss but generally does not produce the curtain effect, instead often resulting in a complete loss of vision in the affected eye.

Thus, the symptomatology and specific description provided by the patient align most clearly with retinal detachment, making it the correct and most probable diagnosis.