Which of these is not an endocrine gland?

Correct Answer: C

Rationale: The correct answer is C, the salivary gland. The salivary gland is not an endocrine gland because it secretes saliva directly into the oral cavity, rather than into the bloodstream like endocrine glands. The pancreas (A) secretes hormones such as insulin and glucagon into the bloodstream. The testes (B) produce hormones like testosterone that are released into the bloodstream. The parathyroid (D) gland secretes parathyroid hormone directly into the bloodstream to regulate calcium levels. In summary, the salivary gland is not considered an endocrine gland because it does not secrete hormones directly into the bloodstream for systemic effects.

Correct Answer: C

Rationale: The correct answer is C: cAMP. Water-soluble hormones, such as adrenaline or glucagon, bind to cell surface receptors and activate a signaling cascade that often involves cAMP as a second messenger. Upon hormone binding, the receptor activates adenylyl cyclase, which converts ATP to cAMP. cAMP then activates protein kinase A, leading to phosphorylation of target proteins, ultimately affecting cellular responses. Explanation for incorrect choices: A: tRNA is involved in protein synthesis, not in signaling pathways of water-soluble hormones. B: ATP is used as an energy source in various cellular processes, but it is not a second messenger in hormone signaling pathways. D: PTH (parathyroid hormone) is a hormone involved in calcium regulation and does not act through cAMP signaling in the same manner as water-soluble hormones.

Correct Answer: D

Rationale: The correct answer is D: Follicle stimulating hormone. It stimulates the testes to produce sperm cells in males and promotes follicle development in females. Growth hormone (A) primarily regulates growth and metabolism. Leutenizing hormone (B) stimulates ovulation in females and testosterone production in males. Prolactin (C) is responsible for milk production in females.

Correct Answer: A

Rationale: The thymus gland is the correct answer because it involutes or shrinks with age, leading to decreased immune function. The parathyroid, thyroid, and pituitary glands do not disappear with old age but may undergo changes in function. The parathyroid regulates calcium levels, the thyroid controls metabolism, and the pituitary gland secretes various hormones. Therefore, the thymus gland is the only gland that disappears during old age, making it the correct answer.

Correct Answer: C

Rationale: The correct answer is C: Aldosterone. The adrenal cortex produces aldosterone, which helps regulate blood pressure and electrolyte balance. Insulin (A) is produced by the pancreas, epinephrine (B) is produced by the adrenal medulla, and glucagon (D) is produced by the pancreas. Aldosterone is the only hormone produced by the adrenal cortex among the options provided, making it the correct answer.