Best disease due to a mutation in the BEST1 (VMD2) is best diagnosed by a pathological:

Best Disease (Juvenile-Onset Vitelliform Dystrophy): It is an autosomal dominant disorder with variable penetrance and expressivity. Onset is usually in childhood. The fundoscopic appearance is a mild pigmentary disturbance within the fovea to the typical vitelliform or "egg yoke" lesion located in the central macula. Visual acuity often remains good, and the ERG is normal. An abnormal electro-oculogram (EOG) is the hallmark of the disease. Ref: Fletcher E.C., Chong N., Augsburger J.J., Correa Z.M. (2011). Chapter 10. Retina. In P. Riordan-Eva, E.T. Cunningham, Jr. (Eds), Vaughan & Asbury's General Ophthalmology, 18e.