A common second messenger used in signaling pathways of water-soluble hormones is

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: 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.

Correct Answer: A

Rationale: The correct answer is A: diabetes mellitus. This condition is characterized by high blood glucose levels due to either insufficient insulin production or the body's inability to use insulin effectively. In diabetes, glucose cannot enter cells for energy, leading to elevated blood sugar levels. Hypoglycemia (B) is low blood sugar levels, opposite of the question. Hypothyroidism (C) is a condition involving an underactive thyroid gland, not directly related to blood glucose levels. Hyperthyroidism (D) is an overactive thyroid gland, also not directly linked to high blood glucose levels.

Correct Answer: B

Rationale: The correct answer is B: The problem is in the kidneys. In this scenario, the low serum osmolality despite water deprivation indicates the kidneys are unable to concentrate urine, leading to excessive loss of water. This condition is known as nephrogenic diabetes insipidus. Choice A is incorrect as pituitary dysfunction would result in high serum osmolality due to lack of ADH. Choice C is incorrect as diabetes mellitus would show high glucose levels. Choice D is incorrect as high ADH in the serum would lead to concentrated urine and high serum osmolality.