A 78-year-old man's painless, complete visual loss is due to what condition?

A painless, complete visual loss in an elderly patient is most indicative of central retinal artery occlusion. This condition occurs when the blood supply to the retina is compromised, often due to embolism or thrombosis. The sudden loss of vision can be profound and complete, characteristically presenting without pain.

In this demographic, factors contributing to central retinal artery occlusion can stem from underlying vascular issues such as hypertension or atherosclerosis, which are common in older adults. Unlike other conditions listed, central retinal artery occlusion typically results in immediate and significant visual impairment, distinguishing it from alternative diagnoses that may present with pain or gradual visual loss.

For instance, acute angle closure glaucoma tends to present with ocular pain and other symptoms like nausea and headache; herpes keratitis usually causes pain and might present with corneal involvement; optic neuritis often presents with pain that precedes or coincides with vision loss, typically affecting younger individuals with demyelinating diseases. Thus, the immediate and painless nature of the visual loss aligns clinically with central retinal artery occlusion as the most plausible cause in this scenario.