Reproduction — Set 5

Correct Answer: C. 28 days

• **28 days** = the average human menstrual cycle lasts 28 days, counted from the first day of one period to the first day of the next, though normal variation ranges from 21 to 35 days. • **Key fact** — The cycle is divided into the follicular phase (days 1–14, ending with ovulation) and the luteal phase (days 15–28); the luteal phase is relatively constant at 14 days. • Ovulation typically occurs around day 14 in a standard 28-day cycle, making this the most fertile window. • 💡 Option A (21 days) is wrong because 21 days is the lower end of the normal range, not the average; Option B (14 days) is wrong because 14 days is the length of the luteal phase, not the whole cycle; Option D (35 days) is wrong because 35 days is at the upper end of the normal range.

Correct Answer: D. GnRH

• **GnRH** = Gonadotropin-Releasing Hormone is secreted by the hypothalamus in pulses and acts on the anterior pituitary to stimulate the synthesis and release of both Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH), which together regulate the gonads. • **Key fact** — GnRH pulses are essential: continuous (non-pulsatile) GnRH actually suppresses FSH/LH, a principle used in GnRH agonist drugs to treat hormone-sensitive cancers. • FSH promotes follicle growth and sperm production, while LH triggers ovulation and testosterone synthesis. • 💡 Option A (STH) is wrong because STH (Somatotropin/Growth Hormone) is secreted by the anterior pituitary and regulates body growth, not gonadal function; Option B (TSH) is wrong because TSH (Thyroid-Stimulating Hormone) acts on the thyroid gland and is controlled by TRH, not GnRH; Option C (ACTH) is wrong because ACTH (Adrenocorticotropic Hormone) stimulates the adrenal cortex and is released in response to CRH.

Correct Answer: A. Navel

• **Navel** = the navel (umbilicus or belly button) is the scar formed after the umbilical cord is cut at birth; the cord previously connected the fetus to the placenta, carrying oxygen and nutrients via the umbilical vein and returning deoxygenated blood via umbilical arteries. • **Key fact** — The umbilical cord contains two arteries and one vein embedded in Wharton's jelly; after birth, the stump dries and falls off within 1–3 weeks, leaving the navel. • The umbilical vein remnant becomes the ligamentum teres (round ligament) of the liver in adults. • 💡 Option B (Sternum) is wrong because the sternum is the breastbone in the centre of the chest with no connection to the umbilical cord; Option C (Liver) is wrong because the liver is an internal organ where the umbilical vein drained prenatally, but the cord itself attached at the abdomen; Option D (Stomach) is wrong because the stomach is a digestive organ unrelated to the umbilical cord attachment.

Correct Answer: A. Sperm

• **Sperm** = the male gamete (spermatozoon) is the haploid reproductive cell produced in the testes that carries the paternal genetic material and fertilizes the egg. • **Key fact** — A sperm cell is about 60 µm long with a head (nucleus + acrosome), a midpiece (mitochondria), and a tail (flagellum); it is the smallest human cell while the ovum is the largest. • The acrosome at the tip of the sperm head contains enzymes that digest the zona pellucida of the egg to allow fertilization. • 💡 Option B (Embryo) is wrong because an embryo is the early multicellular stage of development after fertilization, not a gamete; Option C (Ovum) is wrong because the ovum is the female gamete; Option D (Zygote) is wrong because a zygote is the diploid cell formed immediately after fusion of sperm and ovum.

Correct Answer: C. Catching the released egg

• **Catching the released egg** = fimbriae are finger-like projections at the open end of the fallopian tube (oviduct) that sweep over the ovary surface at ovulation to capture the released secondary oocyte and guide it into the tube. • **Key fact** — Fimbriae are lined with cilia that beat rhythmically, moving the egg along the tube toward the uterus; fertilization normally occurs in the ampulla (the widest part of the fallopian tube). • Damage to fimbriae from infections like PID (pelvic inflammatory disease) can lead to ectopic pregnancies or infertility. • 💡 Option A (Implanting the embryo) is wrong because implantation occurs in the endometrium of the uterus, not at the fimbriae; Option B (Secreting mucus) is wrong because mucus secretion is primarily the function of the cervix and uterine glands; Option D (Producing eggs) is wrong because eggs are produced in the ovary, not the fallopian tube or fimbriae.

Correct Answer: B. Blastocyst

• **Blastocyst** = by days 5–6 after fertilization the embryo has developed into a hollow fluid-filled ball called the blastocyst, consisting of an inner cell mass (which becomes the fetus) and a trophoblast (which becomes the placenta), and it is at this stage that implantation into the endometrium occurs. • **Key fact** — Implantation is completed by day 10–12 post-fertilization; the trophoblast cells invade the endometrium and begin secreting hCG (human chorionic gonadotropin), the hormone detected by pregnancy tests. • 💡 Option A (Zygote) is wrong because the zygote is the single fertilized cell at day 0–1, before any cleavage divisions; Option C (Gastrula) is wrong because gastrulation occurs later (week 3) after implantation has already happened; Option D (Morula) is wrong because the morula is a solid 16-cell ball at day 3–4, just before the blastocyst stage and before reaching the uterine cavity.

Correct Answer: B. Ovary

• **Ovary** = the ovaries are the primary (gonadal) female sex organs because they produce the gametes (oocytes/eggs) and secrete the sex hormones estrogen and progesterone, directly governing reproduction. • **Key fact** — Primary sex organs (gonads) are defined as those that produce gametes; secondary sex organs (vagina, uterus, fallopian tubes) transport or nurture the gametes and embryo; accessory structures (mammary glands) support offspring post-birth. • Each ovary contains thousands of follicles at various developmental stages, and typically one dominant follicle matures and ovulates each month. • 💡 Option A (Vagina) is wrong because the vagina is a secondary sex organ — it is the birth canal and receives sperm but does not produce gametes; Option C (Mammary gland) is wrong because mammary glands are accessory organs for lactation, not gamete production; Option D (Uterus) is wrong because the uterus is a secondary sex organ that nurtures the developing embryo but does not produce eggs.

Correct Answer: A. Vasectomy

• **Vasectomy** = a surgical procedure in which a small section of each vas deferens (the tube carrying sperm from the testes) is cut, tied, or sealed, preventing sperm from reaching the ejaculate and thus achieving permanent male contraception. • **Key fact** — Vasectomy is one of the most effective contraceptive methods (>99%); it does not affect testosterone levels, libido, or sexual function since the testes continue producing hormones normally. • Sperm still produced after vasectomy are simply reabsorbed by the body. • 💡 Option B (Tubectomy) is wrong because tubectomy (or tubal ligation) is the female sterilization procedure that cuts or ties the fallopian tubes; Option C (Hysterectomy) is wrong because hysterectomy is the surgical removal of the uterus in females; Option D (Ligation) is wrong because ligation (tying) is a technique used within both vasectomy and tubectomy, not a standalone procedure name specific to males.

Correct Answer: D. Menstruation

• **Menstruation** = menopause is the permanent cessation of menstrual cycles that occurs when the ovaries stop releasing eggs and dramatically reduce estrogen and progesterone production, typically between ages 45–55 in women. • **Key fact** — Menopause is confirmed after 12 consecutive months without a period; the transition phase before this is perimenopause, during which cycles become irregular and symptoms like hot flashes, mood changes, and bone loss begin. • Falling estrogen after menopause increases the risk of osteoporosis and cardiovascular disease. • 💡 Option A (Circulation) is wrong because circulation (blood flow through the heart and vessels) continues throughout life; Option B (Digestion) is wrong because digestion is an ongoing metabolic process unaffected by menopause; Option C (Respiration) is wrong because cellular respiration and breathing continue as long as the person is alive.

Correct Answer: A. Fertilization

• **Fertilization** = the process in which a haploid sperm fuses with a haploid ovum to form a diploid zygote, restoring the full chromosome number (46 in humans) and initiating embryonic development. • **Key fact** — In humans, fertilization typically occurs in the ampulla of the fallopian tube within 12–24 hours of ovulation; once a sperm penetrates the egg, a cortical reaction prevents polyspermy (entry of additional sperm). • The zygote formed by fertilization undergoes repeated mitotic divisions (cleavage) as it travels to the uterus. • 💡 Option B (Ovulation) is wrong because ovulation is the release of the mature egg from the ovary — a prerequisite for fertilization but not the fusion itself; Option C (Germination) is wrong because germination refers to the sprouting of a seed or spore, a botanical process; Option D (Pollination) is wrong because pollination is the transfer of pollen from anther to stigma in flowering plants, not the fusion of gametes.