## Why ABCA4 gene mutation on chromosome 1p22 is right The clinical presentation—bilateral progressive central vision loss in a teenager with macular atrophy and yellow pisciform flecks (marked **A**)—is pathognomonic for Stargardt disease. The ABCA4 gene encodes a transmembrane transporter in photoreceptor outer-segment discs responsible for clearing N-retinylidene-phosphatidylethanolamine. Mutations in ABCA4 (chromosome 1p22) account for the vast majority of autosomal-recessive Stargardt disease, the most common inherited juvenile macular dystrophy (prevalence 1 in 8,000–10,000). The dark choroid sign on fluorescein angiography is a classic finding due to masking of choroidal fluorescence by lipofuscin-laden RPE. (AAO BCSC Section 12: Retina and Vitreous) ## Why each distractor is wrong - **ELOVL4 gene mutation**: While ELOVL4 mutations do cause autosomal-dominant Stargardt disease (STGD3), this is a rare form and typically presents with a different phenotype. The patient's presentation is consistent with the far more common autosomal-recessive form. - **PROM1 gene mutation**: PROM1 mutations are associated with cone-rod dystrophy and pattern dystrophy, not the classic Stargardt phenotype with pisciform flecks and dark choroid sign. Central vision is typically preserved longer in PROM1-related disease. - **RPE65 gene mutation**: RPE65 mutations cause Leber congenital amaurosis (LCA), which presents with nyctalopia (night blindness) as a primary symptom, early-onset severe vision loss, and a normal fundus in infancy. This does not match the teenager's presentation with macular atrophy and flecks. **High-Yield:** Stargardt disease = ABCA4 mutation → impaired lipofuscin clearance → A2E accumulation in RPE → macular atrophy + yellow pisciform flecks + dark choroid on FA. [cite: AAO BCSC Section 12: Retina and Vitreous]
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