Final answer:
The next best step is to measure the Parathyroid hormone (PTH) level. Hence the correct answer is option D
Step-by-step explanation:
The next best step for the 62-year-old female presenting with progressively severe mid-back pain, fatigue, less mental alertness, high white blood cells, high calcium, high blood urea nitrogen, high creatinine, and low hemoglobin would be Parathyroid hormone (PTH) level measurement.
High levels of calcium and PTH can indicate an overactive parathyroid gland, leading to hyperparathyroidism. This condition can cause symptoms such as bone pain and fatigue. Therefore, measuring the PTH level can help determine the cause of the patient's symptoms and guide further treatment.
Hence the correct answer is option D