Which of the following hormones is increased in response to hypocalcemia due to chronic renal failure?

When hypocalcemia occurs, particularly in the context of chronic renal failure, the parathyroid hormone (PTH) is released as a physiological response to low calcium levels. In patients with chronic renal failure, the kidneys are less able to convert vitamin D to its active form, which is crucial for calcium absorption in the intestines. Consequently, this results in decreased serum calcium levels. The body's response to this decreased calcium is to increase the secretion of PTH from the parathyroid glands.

PTH serves several functions: it increases calcium reabsorption in the kidneys, stimulates the release of calcium from bones, and promotes the activation of vitamin D to enhance gastrointestinal absorption of calcium. This increases serum calcium levels and helps to counteract the effects of hypocalcemia.

In this context, the increase in PTH due to chronic renal failure and resultant hypocalcemia helps to restore calcium balance and mitigate the negative impacts of low calcium levels. Therefore, it is the hormone that is significantly elevated in response to low calcium levels in this setting, making it the correct choice.