A 70-year-old man presents with 4 months of progressive fatigue and lower back pain that acutely worsened after a minor fall while doing yard work yesterday. For the past year, he has had a sensation of incomplete bladder emptying after voiding. Vital signs are within normal limits. Physical examination reveals bilateral paravertebral muscle spasm, severe tenderness over the second lumbar vertebra, and mild tenderness over the lower thoracic vertebrae. Neurologic examination is normal. Laboratory studies show hemoglobin of 10.5 g/dL, alkaline phosphatase of 110 U/L, and serum calcium of 11.1 mg/dL. An x-ray of the skull is shown. Labs: - Hemoglobin: 10.5 g/dL - Alkaline phosphatase: 110 U/L - Calcium: 11.1 mg/dL Which of the following is the most appropriate next step in diagnosis?