Which of the following is the most appropriate next step in managing this patient’s nutritional status?

A 13-year-old white girl has had a recurrent cough that is productive of white sputum for the past 6 months. She is the product of a normal pregnancy, delivered at 37 weeks’ gestation. She also had recurrent upper respiratory infections as a child. She is at the 10th percentile for weight and height, even though her parents and brother are at the 50th percentile for weight and height. She is unhappy with her weight and stature and reports minimal weight gain despite having a large appetite and eating large meals. She also reports chronic large, foul-smelling, fl oating stools. She had her first period 2 years ago but has not had her period in the past 6 months. Her blood pressure is 117/81 mm Hg, pulse is 60/min, respiratory rate is 20/min, and temperature is 38.3°C (101°F). She appears small for her stated age and has digital clubbing. The rest of the physical examination is unremarkable. X-ray of the chest shows hyperinfl ated lungs. Which of the following is the most appropriate next step in managing this patient’s nutritional status?