This highlighted the kidney’s complex role in maintaining systemic 25(OH)D supply and the importance of extrarenal vitamin D activation, although optimal assessment, targets, and supplementation strategies remain highly controversial due to CKD-specific pathophysiology (e.g., megalin loss, impaired uptake, obesity effects) and complex dosing paradoxes. The gene discussed is LRP2; the disease is obesity due to melanocortin 4 receptor deficiency.