GBA1 mutations cause Parkinson's disease partly by creating a lipid-trafficking feedforward loop: reduced GCase activity → glucosylceramide accumulation → retromer dysfunction → further GBA1 trafficking failure. SNX5 (Sorting Nexin 5) senses membrane curvature and may be specifically vulnerable to glucosylceramide-mediated disruption. This challenge asks whether glucosylceramide accumulation specifically impairs SNX5 function and whether substrate reduction therapy (eliglustat) can break the loop. Falsifiable prediction: eliglustat pretreatment of GBA1-N370S iPSC-derived neurons should restore SNX5 membrane association by ≥60% and rescue lysosomal GCase delivery by ≥40%.