A 16-month-old African American female is brought to the tertiary clinic by her parents due to concerns about delaying walking and bowing of her legs. She was born at term via spontaneous vaginal delivery. She is exclusively breastfed and rejects cow's milk and infant formula. Lately, she has become less interested in breastfeeding and is taking supplemental water. She eats a diet consisting mainly of fruits, vegetables, and homemade grains. She has not received any vitamin supplements since birth. The parents report she is otherwise healthy but seems “floppy” compared to her cousins and becomes irritable when bearing weight on her legs. She spends most of her time indoors at a daycare center. There is no family history of bone disease or renal disorders. She has no known drug allergies.

Her height is in the 10th percentile for age, her weight is in the 25th percentile, and her head circumference is in the 25th percentile. She is alert and active, but irritable upon standing.

She is referred by her primary care pediatrician who obtained the following initial laboratory studies:

Calcium: 8.2 mg/dL (8.8-10.8)

Phosphorus: 2.8 mg/dL (4.5-6.5)

Alkaline phosphatase: 850 U/L (150-420)

Parathyroid hormone (PTH): 180 pg/mL (10-65)

Albumin: 4.0 g/dL (3.5-5.0)

Creatinine: 0.3 mg/dL (0.2-0.4)

BUN: 10 mg/dL (5-18)

Electrolytes (Na, K, Cl, HCO3): Normal