Final answer:
The diagnosis most consistent with the symptoms of a cherry red spot, decreased blood vessel size, and painless monocular vision loss is Central retinal artery occlusion (CRAO), characterized by sudden vision loss due to ischemia of the inner retina.
Step-by-step explanation:
The most likely diagnosis for a patient with a cherry red spot on fundoscopic exam, decreased retinal blood vessel size, and painless monocular vision loss is Central retinal artery occlusion (CRAO). The classic fundoscopic appearance of a cherry red spot results from a central retinal artery occlusion. This condition leads to sudden, painless vision loss in one eye because the occlusion results in ischemia to the inner retinal layers.
Over time, the infarcted retina becomes edematous and opaque, save for the fovea, which appears red due to its high concentration of cones and the underlying choroidal circulation showing through - hence the 'cherry red' spot. This symptom distinguishes CRAO from other options such as retinal detachment where the retina appears elevated and undulating, or macular degeneration which usually presents with a gradual loss of central vision and drusen deposits.