Which of the following is true about calcium homeostasis?

Correct Answer: D

Rationale: The correct answer is D: Parathyroid hormone is the single most important regulator of calcium levels in the blood. Parathyroid hormone plays a crucial role in maintaining calcium homeostasis by stimulating bone resorption, increasing calcium reabsorption in the kidneys, and activating vitamin D to enhance calcium absorption in the intestines. It acts to raise blood calcium levels when they are low. Explanation for other choices: A: Increased calcitonin levels actually lower blood calcium levels by promoting calcium deposition in bones. B: High calcium levels do not cause bone resorption; it is actually the opposite as mentioned in choice D. C: Parathyroid hormone does not increase osteoblast activity; it primarily affects osteoclasts to release calcium from bones.

Correct Answer: B

Rationale: The correct answer is B: kidney and blood vessels. ADH, or antidiuretic hormone, acts on the kidney to regulate water reabsorption and maintain fluid balance in the body. It also acts on blood vessels to regulate blood pressure. The other choices are incorrect because ADH does not target the adrenal cortex, liver, pituitary gland, or thyroid gland specifically for its primary functions. ADH primarily acts on the kidney and blood vessels to regulate water balance and blood pressure in the body.

Correct Answer: C

Rationale: The correct answer is C: pituitary dwarfism. Growth hormone deficiency during growing years results in pituitary dwarfism due to stunted growth. Myxedema (choice A) is associated with hypothyroidism, not growth hormone deficiency. Gigantism (choice B) occurs due to excessive growth hormone production in childhood. Acromegaly (choice D) is caused by excess growth hormone in adulthood, not during the growing years. Thus, choice C is the most appropriate based on the impact of growth hormone deficiency on growth during childhood.

Correct Answer: D

Rationale: Correct Answer: D - It is a mineralocorticoid. Rationale: Cortisol is a glucocorticoid, not a mineralocorticoid. Glucocorticoids like cortisol regulate glucose metabolism and counteract inflammation, supporting options A and B. Option C is also incorrect as cortisol promotes the metabolism of glucose over fatty acids.

Correct Answer: D

Rationale: The correct answer is D because testosterone is not controlled by the thyroid gland. Testosterone is primarily produced in the testes and regulated by the hypothalamus and pituitary gland. The other choices (A, B, C) are all true of testosterone. Testosterone is responsible for the growth of the penis and testes (A), development of male secondary sex characteristics (B), and can potentially lead to baldness due to its effects on hair follicles (C). Thyroid gland primarily regulates metabolism and energy levels, not testosterone production.