Soru

A 48-year-old female presents to the clinic with a 6-month history of diffuse, dull bone pain and worsening muscle weakness. She reports increasing difficulty rising from a seated position and climbing the stairs at her home. Her past medical history is notable for a Roux-en-Y gastric bypass performed two years ago for severe obesity. She acknowledges skipping her recommended daily nutritional supplements for the past year. Her vital signs are within normal limits. Physical examination reveals generalized tenderness upon palpation of the sternum and anterior tibias. Neurological examination demonstrates $4/5$ strength in the bilateral hip flexors and quadriceps, with normal deep tendon reflexes. A routine chemistry panel shows a serum calcium of $8.2 \text{ mg/dL}$ (reference: $8.5-10.5 \text{ mg/dL}$) and phosphorus of $2.6 \text{ mg/dL}$ (reference: $2.8-4.5 \text{ mg/dL}$). Which of the following is the most appropriate next diagnostic step?