Seven days after ovulation, pituitary secretion of luteinizing hormone (LH) decreases rapidly. What is the cause of this decrease in secretion?

Correct Answer: D

Rationale: Step 1: After ovulation, the follicle transforms into the corpus luteum. Step 2: The corpus luteum secretes estrogen and progesterone. Step 3: Estrogen and progesterone exert negative feedback on the hypothalamus. Step 4: This inhibits the secretion of GnRH from the hypothalamus. Step 5: Decreased GnRH leads to reduced LH secretion from the pituitary. Therefore, the correct answer is D because the secretion of estrogen and progesterone by the corpus luteum suppresses the hypothalamic secretion of GnRH, resulting in decreased pituitary LH secretion. Summary: A: Incorrect. The anterior pituitary gland's responsiveness to GnRH is not the cause of the decrease in LH secretion. B: Incorrect. Estrogen exerts negative feedback on the hypothalamus, not the pituitary. C: Incorrect. Rise in body temperature does not directly inhibit GnRH secretion

Correct Answer: B

Rationale: The correct answer is B, as RU486 blocks progesterone receptors, preventing progesterone from exerting its effects. Progesterone is crucial for maintaining the uterine lining to support a pregnancy. By blocking progesterone receptors, RU486 causes the uterine lining to break down, leading to the expulsion of the embryo and ultimately causing abortion. Choice A is incorrect because RU486 does not bind to luteinizing hormone receptors or stimulate progesterone secretion. Choice C is incorrect because RU486 does not block the secretion of follicle-stimulating hormone; its mechanism of action is specific to progesterone receptors. Choice D is incorrect because RU486 does not block the effects of oxytocin receptors in the uterine muscle; its primary action is on progesterone receptors.

Correct Answer: A

Rationale: The correct answer is A: An antibody that blocked the effect of human chorionic gonadotropin in the embryo and fetus. Human chorionic gonadotropin (hCG) is crucial for male sexual differentiation. Blocking its effects would lead to the development of female genital organs in pups with XY chromosomes. Estrogen-like compounds (B) and follicle-stimulating hormone (C) are not directly involved in male sexual differentiation. Testosterone (D) is responsible for male sexual development, so giving testosterone would not result in female genital organs in XY pups.

Correct Answer: A

Rationale: Step 1: High levels of gamma radiation cause destruction of germinal epithelium, leading to infertility. Step 2: Normal testosterone levels indicate intact Leydig cells, responsible for testosterone production. Step 3: Normal secretory pattern of gonadotropin-releasing hormone (GnRH) is expected as it regulates pituitary hormone secretion. Step 4: Inhibin is produced by Sertoli cells (part of germinal epithelium) and would be decreased due to destruction. Step 5: Follicle-stimulating hormone (FSH) levels would be elevated, not suppressed, due to lack of inhibin feedback. Step 6: Leydig cells are intact, so Leydig cells are not absent in this scenario.

Correct Answer: B

Rationale: The correct answer is B: At puberty. The monthly ovarian cycle, also known as the menstrual cycle, begins at puberty when a female's body matures and starts ovulating. This is typically around the age of 11-14. At birth (A), females already have all their eggs but do not start ovulating until puberty. Middle age (C) refers to the period around 40-65, which is not when the cycle begins. Menopause (D) is the cessation of the menstrual cycle, marking the end of reproductive years, typically around the age of 45-55.