Best Vitelliform Macular Dystrophy — BEST1 Mutation

An 11-year-old boy presents with progressive difficulty reading the blackboard over the past year. Visual acuity is 6/9 OD and 6/12 OS. Dilated fundus examination reveals a striking bilateral, symmetric, round, well-circumscribed yellow-orange elevated lesion at the fovea with an egg-yolk appearance, approximately 1–2 disc diameters in size. OCT confirms hyperreflective subretinal material at the RPE–photoreceptor interface. The lesion marked **A** in the diagram shows this characteristic vitelliform deposit with an abnormal electrooculography Arden ratio of 1.3 (normal ≥1.85) and a normal full-field electroretinogram. Genetic testing confirms a heterozygous missense mutation in BEST1. Which of the following best describes the molecular basis of the pathology in this condition?