A 2-year-old girl presents with failure to thrive, short stature (height <3rd percentile for age), and biochemical evidence of hypophosphatemic rickets (serum phosphate 2.0 mg/dL, normal 3.5–5.5). Renal function and 1,25-dihydroxyvitamin D levels are normal. Genetic testing confirms X-linked hypophosphatemic rickets (XLH). What is the drug of choice for long-term management to improve growth and prevent skeletal deformities?