Best Vitelliform Macular Dystrophy

A 14-year-old boy presents to the retina clinic with a complaint of gradual blurring of vision in both eyes over the past 2 years. Fundoscopy reveals a homogeneous yellow-orange lesion at the fovea bilaterally, as marked **A** in the diagram. His father has a similar lesion and wears glasses for myopia. Visual acuity is 20/40 in both eyes. Electro-oculography (EOG) shows an Arden ratio of 1.2 (normal >1.8), while full-field electroretinography (ERG) is normal. Which of the following best explains the pathophysiology underlying the lesion marked **A**?

A 12-year-old boy presents to the retina clinic with a complaint of gradual blurring of vision over the past 6 months. Fundoscopy reveals a well-demarcated, round, elevated yellow-orange subretinal deposit centered on the fovea, measuring approximately 1 disc diameter. The structure marked **B** in the diagram shows this classic "egg yolk" appearance. His best-corrected visual acuity is 20/30 in both eyes. An electro-oculogram (EOG) shows an Arden ratio of 1.2 (normal >1.8), while a full-field electroretinogram (ERG) is normal. Which of the following genetic mutations would most likely explain this patient's findings?