Inhibin is

Correct Answer: A

Rationale: Inhibin is secreted by Sertoli cells to inhibit follicle stimulating hormone (FSH) from the anterior pituitary. This is the correct answer because inhibin acts as a negative feedback mechanism to regulate FSH levels in the body. Sertoli cells are specifically involved in spermatogenesis and play a crucial role in regulating FSH secretion through inhibin. Choices B, C, and D are incorrect because inhibin does not inhibit testosterone secretion or act on Leydig cells. It specifically targets FSH secretion to regulate the process of spermatogenesis.

Correct Answer: D

Rationale: The correct answer is D: menstruation. Day 1 of the menstrual cycle is the first day of menstruation, which typically lasts around 3-7 days. Menstruation is the shedding of the uterine lining when pregnancy has not occurred. This marks the beginning of a new cycle. Menopause (A) is the cessation of menstruation and fertility typically occurring in women in their 40s-50s. Ovulation (B) is the release of an egg from the ovary, usually occurring around day 14 of the cycle. Implantation (C) is the attachment of a fertilized egg to the uterus, which occurs around 6-10 days after ovulation.

Correct Answer: C

Rationale: The correct answer is C because the endocrine role of the testicle primarily involves the secretion of testosterone. Testosterone is the main androgenic hormone produced by the testicles, which plays a crucial role in various physiological functions such as the development of male reproductive organs, sperm production, and secondary sexual characteristics. Choice A is incorrect because while interstitial cells (Leydig cells) are involved in the production of testosterone, the endocrine role of the testicle is not limited to these cells alone. Choice B is incorrect as it is too general. While androgenic hormones are indeed secreted by the testicles, testosterone specifically is the main androgenic hormone produced by them. Choice D is incorrect as the maturation of sex organs is primarily regulated by the action of testosterone but is not the sole endocrine role of the testicle.

Correct Answer: D

Rationale: The correct answer is D: seminal vesicles. Seminal vesicles are accessory glands that produce a significant portion of the fluid that makes up semen. They contribute nutrients and substances that support sperm viability and motility. Choices A, B, and C are incorrect as they are structures of the penis involved in erection and urination, not accessory glands of the male reproductive system.

Correct Answer: A

Rationale: Step-by-step rationale for choice A being correct: 1. Sperm cells need to be mobile to reach the egg for fertilization. 2. Seminal fluid provides the environment for sperm motility. 3. Sperm cells acquire motility in the seminal fluid due to specific nutrients and ions present. Summary of why other choices are incorrect: - Choice B: Being microscopic is a general characteristic of sperm cells but not specific to their mobility. - Choice C: Sperm cells are stored in the epididymis, not the "crotum." - Choice D: Sperm cells are all of the same kind, not three different kinds.