Two days before the onset of menstruation, secretions of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) reach their lowest levels. What is the cause of this low level of secretion?

Correct Answer: D

Rationale: The correct answer is D because two days before menstruation, the corpus luteum secretes estrogen, progesterone, and inhibin, which collectively suppress the hypothalamus's secretion of GnRH and the pituitary gland's secretion of FSH and LH. This hormonal feedback loop helps prepare the body for menstruation. A: The anterior pituitary gland becoming unresponsive to GnRH does not explain the low FSH and LH levels. B: Estrogen from developing follicles typically exerts a positive feedback on the hypothalamus, not inhibitory feedback. C: The rise in body temperature does not directly impact FSH and LH levels; it is more related to ovulation.

Correct Answer: A

Rationale: The correct answer is A: uterine (fallopian) tubes. These tubes receive the ovulated oocyte from the ovary and provide the site for fertilization. The other choices are incorrect because: B: The vagina is the birth canal and site for sperm deposition, not fertilization. C: The ductus deferens is part of the male reproductive system, not involved in fertilization. D: The uterus is where the fertilized egg implants and develops into a fetus, it is not the site for fertilization.

Correct Answer: A

Rationale: The correct answer is A: spermatogonium. Spermatogonium is the primitive stem cell of spermatogenesis located at the periphery of seminiferous tubules. It undergoes mitosis to produce more spermatogonia or differentiate into primary spermatocytes. Spermatid and secondary spermatocyte are later stages in spermatogenesis, not the primitive stem cell. Primary spermatocyte is the immediate progeny of spermatogonium following differentiation.

Correct Answer: D

Rationale: The correct answer is D: LH triggers ovulation in a female. LH is a hormone released by the pituitary gland that stimulates the release of an egg from the ovary during the menstrual cycle. This process is known as ovulation. It is a crucial step in the reproductive cycle of females. A: LH does not promote the enlargement of female breasts. Breast growth is primarily regulated by estrogen and progesterone. B: LH does have a significant effect on females, particularly in the reproductive system. C: While LH does contribute to the development of secondary sex characteristics in females, its main role is in triggering ovulation.

Correct Answer: B

Rationale: The correct answer is B: oxytocin. Braxton Hicks contractions are irregular, painless contractions of the uterus that occur throughout pregnancy. Oxytocin is the hormone responsible for stimulating contractions during labor. Therefore, the presence of Braxton Hicks contractions indicates the preparation of the uterus for labor under the influence of oxytocin. Choices A, C, and D are incorrect as they do not directly relate to the physiological mechanism of Braxton Hicks contractions.