Most Common cause gradual vision loss in Diabetic Retinopathy:
Macular edema is the most common cause of gradual vision loss. Explanation: Since retinopathy is asked answer would be macular edema, but if diabetes in general would have been asked then answer will be Cataract. Remember most common cause of sudden vision loss in adult with proliferative diabetic retinopathy is vitreous hemorrhage.
Cataract
Macular edema
Glaucoma
Keratoconus
Retina