A 74-year-old woman has metastatic bone disease on x-ray. Which of the following mediators is least likely to be involved?

A whole host of locally produced hormones and cytokines, as well as ectopically produced hormones, are implicated in local osteoclastic hypercalcemia. Parathormone-related protein is often elevated in malignant hypercalcemia (with or without bony metastases), but ectopic PTH production in malignancy is quite rare.