A 18-month-old girl from an urban slum in Delhi is brought to the clinic by her father with a complaint of slow growth. She was born at term with a birth weight of 2.6 kg. Current measurements: weight 8.5 kg, length 75 cm, head circumference 45 cm. On examination, she is alert and playful, with no dysmorphic features. Skin turgor is normal, and there is no edema. Developmental milestones are delayed (speaks only 5–6 words, walks with support). When plotted on the WHO growth chart, her weight-for-length is at the 15th percentile, length-for-age is at the 8th percentile, and weight-for-age is at the 10th percentile. Her hemoglobin is 8.2 g/dL. What is the most appropriate next step in the evaluation?