Question

Serum calcium levels are usually normal in spite of suboptimal calcium absorption and Vitamin D deficiency. Serum calcium is being maintained A. by low parathyroid hormone (PTH) levels inhibiting calcium excretion. B. by an increase in calcitonin. C. by increased bone resorption stimulated by clevated PTH. D. because PTH stimulates calcium absorption from the stomach. E. by lack of 1,25 -dihydroxy Vitamin D which prevents bone from taking calcium from blood.

Short answer

Short Answer: Serum calcium levels are maintained by increased bone resorption stimulated by elevated parathyroid hormone (PTH).

Step-by-step solution

Understanding the role of parathyroid hormone (PTH) in calcium homeostasis

Parathyroid hormone (PTH) is produced by the parathyroid glands and plays a key role in maintaining calcium homeostasis. When serum calcium levels drop, PTH is released and acts on the bones, kidneys, and intestines to help increase serum calcium levels. In the bones, PTH increases bone resorption, releasing calcium from the bones into the bloodstream. In the kidneys, PTH promotes calcium reabsorption, reducing the amount of calcium excreted in the urine. In the intestines, PTH stimulates the conversion of Vitamin D to its active form, calcitriol, which in turn increases calcium absorption.

Understanding the role of calcitonin in calcium homeostasis

Calcitonin is a hormone secreted by the thyroid gland that helps to regulate calcium levels in the blood. It acts in opposition to PTH and is released when serum calcium levels are high. Calcitonin promotes calcium deposition in bones and reduces the activity of osteoclasts, the cells responsible for bone resorption. It also inhibits calcium reabsorption in the kidneys, promoting calcium excretion.

Understanding the role of Vitamin D in calcium homeostasis

Vitamin D is essential for calcium absorption in the intestines and helps to maintain normal serum calcium levels. It is primarily synthesized in the skin in response to sunlight exposure but can also be obtained from the diet. In its active form, calcitriol, Vitamin D stimulates the absorption of calcium from the intestines into the bloodstream.

Analyzing the answer choices

A. Low PTH levels would not inhibit calcium excretion; instead, they would promote it. PTH levels would be elevated to maintain serum calcium levels. B. An increase in calcitonin would lower serum calcium levels rather than maintain them. C. Increased bone resorption stimulated by elevated PTH would help maintain serum calcium levels, making this the correct answer. D. PTH does not directly stimulate calcium absorption from the stomach; it acts on the intestines via the conversion of Vitamin D to its active form. E. Lack of 1,25-dihydroxy Vitamin D would make it more difficult to maintain serum calcium levels, as it is essential for intestinal calcium absorption.

Conclusion

The correct answer is C: Serum calcium levels are maintained by increased bone resorption stimulated by elevated parathyroid hormone (PTH).