A 66-year-old woman with a history of breast cancer treated 5 years ago presents with persistent back pain for 2 months, worse at night and not relieved by simple analgesia. Neurological examination of the lower limbs is currently normal. Her inflammatory markers (ESR, CRP) are normal, but her serum calcium level is elevated at 2.95 mmol/L (Normal range approx 2.2-2.6 mmol/L). What is the most important investigation to perform urgently?