Which of the following hormones exerts negative feedback control on the secretion of ACTH?

Correct Answer: C

Rationale: The correct answer is C: Cortisol. Cortisol exerts negative feedback control on the secretion of ACTH by inhibiting the release of corticotrophin-releasing hormone (CRH) and ACTH from the hypothalamus and pituitary gland, respectively. This feedback loop helps regulate the body's stress response and maintain hormone levels within a normal range. A: PTH (Parathyroid Hormone) does not directly regulate the secretion of ACTH. B: Glucagon regulates blood sugar levels and does not directly affect ACTH secretion. D: Corticotrophin-releasing hormone (CRH) is the hormone that stimulates the release of ACTH, so it does not exert negative feedback control on ACTH secretion.

Correct Answer: C

Rationale: The correct answer is C: Pineal gland. The pineal gland is located in the brain and is often referred to as the "biological clock" due to its role in regulating the sleep-wake cycle through the secretion of melatonin. The posterior pituitary (choice A) is responsible for releasing hormones produced by the hypothalamus, not related to the circadian rhythm. The hypothalamus (choice B) is involved in regulating various physiological processes but is not directly associated with the circadian rhythm. The thymus (choice D) plays a role in the immune system, not in regulating the biological clock. Thus, the pineal gland is the correct answer as it specifically controls the circadian rhythm.

Correct Answer: A

Rationale: The correct answer is A: Thyroid. The thyroid gland produces thyroid hormones T3 and T4, which regulate the body's metabolic rate. These hormones control the basal metabolic rate (BMR) by influencing the rate at which cells use energy. The other choices, B: Parathyroid, C: Testes, and D: Pancreas, do not directly control BMR. Parathyroid regulates calcium levels, Testes produce hormones related to reproduction, and Pancreas regulates blood sugar levels through insulin and glucagon secretion. Thyroid is the only gland among the choices that specifically influences BMR.

Correct Answer: A

Rationale: The correct answer is A: sweat gland. Endocrine glands secrete hormones directly into the bloodstream, affecting distant target organs. Sweat glands, on the other hand, are exocrine glands that release sweat onto the skin surface to regulate body temperature. The adrenal glands, thyroid gland, and pituitary gland are all endocrine glands that secrete hormones, making them the correct choices. A sweat gland does not secrete hormones into the bloodstream, so it does not qualify as an endocrine gland.

Correct Answer: C

Rationale: Rationale: 1. Oxytocin is released during childbirth to stimulate uterine contractions. 2. As contractions increase, oxytocin secretion is further stimulated in a positive feedback loop. 3. This positive feedback mechanism amplifies the contractions until delivery. 4. Calcitonin regulates calcium levels, histamine is involved in allergic responses, and melatonin regulates sleep-wake cycles. Summary: - Calcitonin, histamine, and melatonin do not exhibit positive feedback regulation like oxytocin during childbirth.