A 55-year-old woman with fatigue, weakness, and renal calculi is found to have hypercalcemia. What is the most likely diagnosis?

The most likely diagnosis in this case is chief cell adenoma of the parathyroid gland, commonly known as primary hyperparathyroidism. This condition is characterized by excessive production of parathyroid hormone (PTH) due to a benign tumor of the chief cells in the parathyroid glands. The increased levels of PTH lead to elevated calcium levels in the blood (hypercalcemia) as a result of increased renal tubular reabsorption of calcium, increased intestinal absorption of calcium (due to activation of vitamin D), and enhanced mobilization of calcium from the bones.

In this scenario, the combination of hypercalcemia, fatigue, weakness, and the presence of renal calculi (kidney stones) strongly suggests primary hyperparathyroidism. The renal calculi are a direct consequence of the increased calcium levels, which can lead to calcium precipitation in the kidneys.

Other conditions, such as secondary hyperparathyroidism or familial hypocalciuric hypercalcemia, would lead to different clinical presentations and laboratory findings. For instance, secondary hyperparathyroidism typically occurs in response to low calcium levels and vitamin D deficiency, where there is a compensatory increase in parathyroid hormone to restore calcium levels. Familial (benign) hyp