Final answer:
Increased serum calcium levels lead to a decrease in the secretion of parathyroid hormone (PTH) due to a negative feedback loop.
Step-by-step explanation:
The imbalance that lessens the rate of secretion of parathyroid hormone (PTH) is A. Increased serum calcium levels. PTH is secreted by the parathyroid glands in response to low blood calcium levels. Its main roles are to increase blood calcium levels by stimulating the conversion of calcidiol to the active form of vitamin D (calcitriol), promoting calcium release from bones, and reducing the excretion of calcium by the kidneys. A negative feedback loop involving rising blood calcium levels inhibits the release of PTH, meaning that when serum calcium levels are high, PTH secretion is decreased.