The second-messenger mechanism of hormone action operates by ________.

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: 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.

Correct Answer: D

Rationale: The correct answer is D: adrenal cortex. ACTH stimulates the adrenal cortex to produce and release its hormones, including cortisol. The anterior pituitary produces ACTH, but it does not release hormones itself. The adrenal medulla is primarily stimulated by the sympathetic nervous system, not ACTH. The kidneys are not directly stimulated by ACTH to produce and release hormones. Therefore, the adrenal cortex is the correct choice as it directly responds to ACTH stimulation.

Correct Answer: D

Rationale: The correct answer is D: adrenal gland. The adrenal gland secretes cortisol and aldosterone. Cortisol is essential for regulating metabolism, immune response, and stress. Aldosterone helps regulate blood pressure and electrolyte balance. The other choices are incorrect because the thyroid gland (A) secretes hormones like thyroid hormone and calcitonin, the parathyroid gland (B) secretes parathyroid hormone, and the pancreas (C) secretes insulin and glucagon. Only the adrenal gland (D) secretes cortisol and aldosterone, making it the correct answer.