Reproduction — Set 4

Correct Answer: B. Midpiece

• **Midpiece** = the middle segment of sperm that is densely packed with mitochondria, which produce ATP through cellular respiration to power the beating of the flagellum. • **Key fact** — Mitochondria in the midpiece form a spiral sheath around the axoneme, ensuring a continuous energy supply during the long journey to the egg. • Without this ATP, the tail (flagellum) cannot whip back and forth, making motility impossible. • 💡 Option A (Head) is wrong because the head contains the nucleus and acrosome, not the energy-producing organelles; Option C (Neck) is wrong because the neck is just the connecting segment between head and midpiece; Option D (Tail) is wrong because the tail is the locomotive structure that uses ATP but does not produce it.

Correct Answer: C. Menstruation

• **Menstruation** = the monthly breakdown and shedding of the endometrium (uterine lining) that occurs when a released egg is not fertilized and progesterone levels drop. • **Key fact** — The shedding typically lasts 3–7 days and results in blood loss of approximately 30–80 mL per cycle. • The drop in estrogen and progesterone at the end of the luteal phase triggers vasoconstriction and subsequent tissue breakdown. • 💡 Option A (Ovulation) is wrong because ovulation is the release of an egg from the ovary, not the shedding of the lining; Option B (Conception) is wrong because conception refers to the fertilization of the egg; Option D (Implantation) is wrong because implantation is the embedding of a fertilized egg into the uterine wall.

Correct Answer: A. Inner lining of the uterus

• **Inner lining of the uterus** = the endometrium is the mucous membrane that lines the inside of the uterus and undergoes cyclic changes under hormonal influence to support potential implantation. • **Key fact** — It has two layers: the functional layer (shed each month during menstruation) and the basal layer (remains and regenerates the functional layer). • In pregnancy, the endometrium thickens further to nourish the embryo before the placenta forms. • 💡 Option B (Outer layer of the heart) is wrong because the outer covering of the heart is the pericardium; Option C (Covering of the brain) is wrong because the brain is covered by meninges (dura mater, arachnoid, pia mater); Option D (Muscle of the arm) is wrong because arm muscles are simply skeletal muscle tissue with no relation to the uterus.

Correct Answer: A. Y chromosome

• **Y chromosome** = the Y chromosome carries the SRY (sex-determining region Y) gene, which triggers testis development in the embryo and thus determines the male sex. • **Key fact** — Males have an XY karyotype while females have XX; since sperm can carry either X or Y, the father's gamete determines the sex of the offspring. • The SRY gene activates a cascade leading to testosterone production, which masculinises the developing fetus. • 💡 Option B (Chromosome 21) is wrong because it is an autosome; trisomy 21 causes Down syndrome, not sex determination; Option C (X chromosome) is wrong because a single X (with no Y) results in a female or Turner syndrome; Option D (Chromosome 18) is wrong because it is an autosome and trisomy 18 causes Edwards syndrome.

Correct Answer: D. Amniotic fluid

• **Amniotic fluid** = the clear, slightly yellowish liquid enclosed within the amniotic sac that cushions the fetus, maintains temperature, allows movement, and supports lung development. • **Key fact** — By the third trimester, amniotic fluid volume reaches approximately 800–1000 mL; the fetus constantly swallows and excretes it to practise breathing and digestion. • Abnormally low fluid (oligohydramnios) or high fluid (polyhydramnios) can indicate fetal abnormalities. • 💡 Option A (Cerebrospinal fluid) is wrong because CSF surrounds the brain and spinal cord, not the fetus; Option B (Lymph) is wrong because lymph flows in lymphatic vessels throughout the body; Option C (Bile) is wrong because bile is a digestive fluid produced by the liver and stored in the gallbladder.

Correct Answer: A. Oogenesis

• **Oogenesis** = the process of formation and maturation of the female gamete (ovum) within the ovary, involving meiotic divisions that begin before birth and complete only at fertilization. • **Key fact** — Oogenesis produces one functional ovum and two or three polar bodies from each primary oocyte; the unequal cytoplasm distribution ensures the egg retains nutrients. • Females are born with all their primary oocytes (about 1–2 million), of which only around 400 are ever ovulated. • 💡 Option B (Spermatogenesis) is wrong because that is the production of male gametes (sperm) in the testes; Option C (Mitosis) is wrong because mitosis produces genetically identical somatic cells, not gametes; Option D (Metamorphosis) is wrong because metamorphosis refers to transformation in body form, as seen in insects and amphibians.

Correct Answer: B. Estrogen

• **Estrogen** = a steroid hormone produced mainly by the growing follicle in the ovary that stimulates proliferation and thickening of the endometrium during the follicular phase of the menstrual cycle. • **Key fact** — Rising estrogen also triggers the LH surge that causes ovulation, making it the central hormone of the first half of the cycle. • After ovulation, progesterone (from the corpus luteum) takes over to mature and stabilize the thickened lining for implantation. • 💡 Option A (Glucagon) is wrong because glucagon is a pancreatic hormone that raises blood glucose; Option C (Testosterone) is wrong because testosterone is the primary male sex hormone that promotes male secondary characters; Option D (Adrenaline) is wrong because adrenaline (epinephrine) is the fight-or-flight hormone secreted by the adrenal medulla.

Correct Answer: B. Cervix

• **Cervix** = the cylindrical, narrow inferior portion of the uterus that projects into the vagina and serves as the gateway between the uterine cavity and the vaginal canal. • **Key fact** — The cervix produces mucus that changes in consistency across the menstrual cycle: thin and slippery at ovulation to facilitate sperm entry, thick and sticky at other times to block pathogens. • During labour, the cervix dilates to approximately 10 cm to allow the baby to pass through. • 💡 Option A (Vulva) is wrong because the vulva refers to the entire external female genitalia, not the internal narrow neck of the uterus; Option C (Hymen) is wrong because the hymen is a thin membrane partially covering the vaginal opening; Option D (Clitoris) is wrong because the clitoris is a small erectile organ at the top of the vulva involved in sexual response.

Correct Answer: D. Penis

• **Penis** = the external male organ that serves dual functions — it is the passage for urine from the bladder and for semen during ejaculation, and it contains erectile tissue (corpora cavernosa and corpus spongiosum). • **Key fact** — The penis and scrotum are the two external male reproductive organs; the scrotum houses the testes outside the body cavity to maintain the lower temperature required for spermatogenesis. • 💡 Option A (Seminal vesicle) is wrong because seminal vesicles are internal glands that contribute fructose-rich fluid to semen; Option B (Prostate) is wrong because the prostate is an internal gland surrounding the urethra that secretes alkaline fluid; Option C (Ejaculatory duct) is wrong because the ejaculatory duct is an internal passage formed by the union of the vas deferens and seminal vesicle duct.

Correct Answer: C. One

• **One** = oogenesis is an unequal meiotic division that yields only one large functional egg (secondary oocyte → ovum) and polar bodies that degenerate, ensuring maximum cytoplasm and nutrients go to a single egg. • **Key fact** — Meiosis I produces a secondary oocyte and the first polar body; Meiosis II (completed only at fertilization) produces the mature ovum and the second polar body — giving 1 egg + 2–3 polar bodies total. • This contrasts sharply with spermatogenesis, where one primary spermatocyte produces four functional sperm. • 💡 Option A (Two) is wrong because two cells do result from meiosis I but only one is functional (the other is the first polar body); Option B (Three) is wrong because three is the number of polar bodies, not functional eggs; Option D (Four) is wrong because four functional gametes are produced in spermatogenesis, not oogenesis.