What stimulates the release of PTH from the parathyroid gland?

Correct Answer: D

Rationale: The correct answer is D: low levels of calcium in the blood. When calcium levels drop, the parathyroid gland releases parathyroid hormone (PTH) to increase calcium levels in the blood. This is a negative feedback mechanism to maintain calcium homeostasis. A: TSH from the posterior pituitary gland is incorrect because TSH is involved in regulating thyroid function, not PTH release. B: High levels of calcium in the blood would not stimulate PTH release as the body aims to lower, not raise, calcium levels. C: Calcitonin from the anterior pituitary gland is incorrect because calcitonin is released by the thyroid gland and works to lower blood calcium levels, opposing the action of PTH.

Correct Answer: D

Rationale: The correct answer is D because the second-messenger mechanism involves the hormone binding to specific receptors on the cell surface, activating G proteins, which then trigger the production of cyclic adenosine monophosphate (cAMP) inside the cell. cAMP acts as a secondary messenger to relay the hormone's signal and initiate a cellular response. Synthesizing more hormone than needed (A) is not related to the mechanism. Increasing basal metabolic rate (B) is not a direct result of the second-messenger system. Not responding to feedback (C) contradicts the regulatory nature of hormone actions.

Correct Answer: C

Rationale: The correct answer is C: An amino acid derivative can be a hormone. This statement is correct because many hormones are derived from amino acids, such as thyroid hormones (derived from tyrosine) and epinephrine (derived from phenylalanine). Hormones can be derived from various sources, including amino acids, peptides, lipids, and even gases. Explanation of other choices: A: Prostaglandins are not biologically active peptides; they are lipid compounds derived from fatty acids. B: Modified cholesterol does not form the main structural component of peptone hormones; peptides are the main structural components of peptide hormones. D: Testosterone is a steroid hormone and not an example of a paracrine, which is a type of chemical messenger that acts on nearby cells.

Correct Answer: D

Rationale: The correct answer is D: excessively high blood iodine concentrations. High blood iodine levels do not inhibit TSH release; they actually stimulate it. Growth hormone-inhibiting hormone (GHIH), rising levels of glucocorticoids, and somatostatin are all factors that inhibit TSH release. GHIH inhibits TSH release by acting on the pituitary gland. Glucocorticoids have a negative feedback effect on the hypothalamus and pituitary gland, reducing TSH release. Somatostatin inhibits the release of various hormones, including TSH.

Correct Answer: C

Rationale: The correct answer is C: Pituitary gland. The pituitary gland is an endocrine gland located at the base of the brain, below the hypothalamus. It secretes various hormones that regulate important functions in the body. The other choices are incorrect because: A: Adrenal gland - located on top of the kidneys, not in the brain. B: Pancreas - located in the abdomen, not in the brain. D: Parathyroid glands - located in the neck, not in the brain.