Final answer:
Primary hyperparathyroidism is most commonly caused by tumors of the parathyroid gland, resulting in overproduction of parathyroid hormone and high serum calcium levels.
Step-by-step explanation:
Primary hyperparathyroidism is most commonly caused by tumors of the parathyroid gland. These tumors result in an overproduction of parathyroid hormone (PTH), which leads to increased reabsorption of calcium from the bones and into the bloodstream. This causes bone destruction, kidney stones, and high serum calcium levels.