What is the primary cause of hypercalcemia in a 55-year-old woman with decreased phosphorus concentration?

The primary cause of hypercalcemia in this patient is most appropriately attributed to primary hyperparathyroidism. This condition is frequently associated with elevated calcium levels due to an excess of parathyroid hormone (PTH), which facilitates increased calcium resorption from the bones, enhanced intestinal absorption of calcium, and decreased renal excretion of calcium. In cases of primary hyperparathyroidism, it is common to find low or normal phosphorus levels in conjunction with the hypercalcemia. This is because PTH also stimulates the renal excretion of phosphate, which leads to lower serum phosphate concentrations.

In contrast, malignancy typically causes hypercalcemia through mechanisms such as osteolytic metastases or paraneoplastic syndromes, often with normal or elevated phosphorus levels. Secondary hyperparathyroidism results from low calcium levels (often due to renal failure or vitamin D deficiency), leading to increased PTH secretion and is associated with high phosphate levels as well, contrary to the decreased phosphorus concentration observed in this case. Renal failure generally presents with elevated phosphate levels due to the kidneys' inability to excrete phosphate effectively, and hypercalcemia can occur, but it typically is not the primary cause in the context of decreased phosphate levels. Hence, the best interpretation of the clinical