A 3-year-old boy is brought to the pediatric clinic by his mother with a complaint of poor growth. On examination, his height is 85 cm and weight is 11 kg. Using the WHO growth charts for Indian children, he plots below the 3rd percentile for height and weight. The child appears clinically well with no dysmorphic features or organomegaly. Which is the most appropriate next investigation to differentiate constitutional delay of growth from pathological short stature?