While attending the morning report, the discussion was about a 60-year-old patient who presented with low level of consciousness and was found to have elevated calcium. He was managed with Intravenous fluids, calcitonin, Bisphosphonate intravenously and when he started to pass urine, he was given furosemide to help getting rid of Calcium. The patient is stable now. They expected malignancy is the cause for this dramatic presentation. What is the key investigation you have to do?