A 6-year-old boy from a low-income urban area presents with delayed motor milestones, bowing of legs, and frontal bossing. Serum calcium is 7.8 mg/dL, phosphate is 3.2 mg/dL, alkaline phosphatase is 280 IU/L, and PTH is markedly elevated. Chest X-ray shows rachitic rosary and metaphyseal widening. Which investigation would most specifically differentiate nutritional rickets from vitamin D-dependent rickets type 1?