Which of the following explains the suppression of lactation during pregnancy?

Correct Answer: D

Rationale: Although the high circulating levels of estrogen stimulate prolactin secretion during pregnancy, the action of prolactin on the breast is inhibited by progesterone and estrogen. After parturition, progesterone and estrogen levels decrease dramatically. Prolactin can then interact with its receptors in the breast, and lactation proceeds if initiated by suckling.

Correct Answer: C

Rationale: Steroids are lipophilic and diffuse out immediately after synthesis.

Correct Answer: D

Rationale: This describes the cAMP second-messenger system used by many hydrophilic hormones.

Correct Answer: D

Rationale: The suprachiasmatic nucleus (SCN) generates circadian rhythms via clock proteins.

Correct Answer: D

Rationale: LH and FSH from the anterior pituitary stimulate gonadal function.