The main structures in erectile tissue responsible for maintaining an erection are the:

Correct Answer: C

Rationale: The correct answer is C: helicine arteries. Helicine arteries are the main structures in erectile tissue responsible for maintaining an erection. They dilate in response to signals from the nervous system, allowing increased blood flow into the penis, leading to an erection. Pacinian corpuscles (A) are sensory receptors for touch and pressure, not involved in erectile function. Peripheral nerves (B) transmit signals between the brain and the penis but are not directly responsible for maintaining an erection. Acrosome phase (D) is a term related to sperm structure and not relevant to erectile tissue function.

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: D

Rationale: Step-by-step rationale: 1. After birth, the foramen ovale closes, reducing flow (A) 2. Pressure in the right atrium decreases due to closing of fetal shunts (B) 3. Ductus arteriosus closes, reducing flow (C) 4. Aortic pressure increases as systemic circulation begins (D) Summary: A, B, and C decrease after birth due to closure of fetal shunts, while D increases as systemic circulation takes over, making it the correct choice.

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.