A 4-year-old boy is brought to the pediatric emergency department by his mother who noticed a firm, non-tender abdominal mass while bathing him 2 weeks ago. Abdominal ultrasound reveals a large heterogeneous mass confined to the left kidney with no evidence of inferior vena cava thrombus. The mass shown as **A** in the diagram exhibits the characteristic tan-fleshy appearance with areas of hemorrhage and necrosis. Which of the following best describes the embryologic origin and pathogenesis of this lesion?