A 54-year-old woman has severe back pain and elevated serum calcium. What is the least likely mechanism for her hypercalcemia?

• A. Excessive parathyroid hormone production
• B. Increased fractional calcium gastrointestinal absorption
• C. Local interleukin-1 effects
• D. Excessive hormone-related protein production

Answer: (A)

In this scenario, the woman's symptoms of severe back pain and elevated serum calcium levels suggest a potential condition such as malignancy-related hypercalcemia. In this context, the least likely mechanism for her hypercalcemia would be excessive parathyroid hormone production.

In primary hyperparathyroidism, which involves excessive parathyroid hormone (PTH), the mechanism typically leads to increased bone resorption, renal tubular calcium reabsorption, and enhanced intestinal absorption of calcium through the action of vitamin D. However, in cases of malignancy, particularly with certain types of cancers, hypercalcemia can occur through alternative mechanisms that are more common than primary hyperparathyroidism.

For instance, increased fractional gastrointestinal calcium absorption is often associated with enhanced vitamin D production or intake, which can occur in some types of cancers. Local interleukin-1 effects can lead to bone resorption and hypercalcemia through a process known as osteolytic metastasis, where tumors secrete cytokines that stimulate osteoclast activity, consequently releasing more calcium into the bloodstream. Excessive hormone-related protein production, particularly parathyroid hormone-related peptide (PTHrP), is a frequent cause of hypercalcemia in cancer patients, mimicking the effects of P