A 32-year-old obese multigravida (G3P2, BMI 33 kg/m²) with gestational diabetes mellitus diagnosed at 24 weeks is reviewed at 34 weeks' gestation. Despite insulin therapy targeting fasting glucose < 5.3 mmol/L, her home monitoring shows fasting sugars of 5.8–6.3 mmol/L. Growth scan shows head circumference on the 60th centile but the structure marked **A** is above the 95th centile, with increased subcutaneous fat and hepatomegaly. Amniotic fluid index is 28 cm. Estimated fetal weight is 4200 g at 34 weeks, projected to exceed 4500 g at term. Which of the following BEST explains the disproportionate enlargement of the structure marked **A** in this case of diabetic macrosomia?