A 3-year-old boy is brought to the pediatric clinic by his mother with concerns about short stature. On examination, his height is 85 cm (below the 3rd percentile for age). His weight is 12 kg (10th percentile), and head circumference is 48 cm (5th percentile). Birth history reveals normal delivery at term with birth weight 2.8 kg. The child has normal developmental milestones and no dysmorphic features. Which investigation is most appropriate to establish the diagnosis of constitutional growth delay versus pathological short stature?