A 38-year-old man who has galactorrhea is found to have a prolactinoma. His physician treats him with bromocriptine, which eliminates the galactorrhea. The basis for the therapeutic action of bromocriptine is that it

Correct Answer: C

Rationale: Bromocriptine is a dopamine agonist. The secretion of prolactin by the anterior pituitary is tonically inhibited by the secretion of dopamine from the hypothalamus. Thus, a dopamine agonist acts just like dopamine "it inhibits prolactin secretion from the anterior pituitary.

Correct Answer: D

Rationale: Peptides (e.g., insulin) and catecholamines (e.g., epinephrine) are polar and water-soluble.

Correct Answer: D

Rationale: Hydrophilic hormones use membrane receptors and second messengers, not intracellular receptors.

Correct Answer: B

Rationale: Hormone secretion adjusts dynamically, not at a constant rate, to maintain homeostasis.

Correct Answer: B

Rationale: Melatonin neutralizes free radicals, protecting cells from oxidative damage.