- A researcher adds human chorionic gonadotropin (hCG) to a culture of testicular tissue obtained from a healthy 25-year-old man. After incubation, testosterone concentration in the culture medium rises markedly. Which of the following testicular cell types is primarily responsible for this change, and which pituitary hormone normally stimulates that cell type in vivo?
Answer: Leydig cell — luteinizing hormone
This question tests your understanding of testicular hormone production and the hypothalamic-pituitary-gonadal axis. When you see hCG causing testosterone production, think about which testicular cells produce testosterone and what normally stimulates them.
Testosterone is produced by Leydig cells (also called interstitial cells) located in the connective tissue between the seminiferous tubules. These cells respond to luteinizing hormone (LH) from the anterior pituitary by producing testosterone. Human chorionic gonadotropin (hCG) can stimulate testosterone production because it has LH-like activity - both hCG and LH bind to the same LH/hCG receptor on Leydig cells.
Choice A is correct because Leydig cells produce testosterone in response to LH stimulation. The dramatic testosterone increase after hCG addition confirms these cells are responsible.
Choice B incorrectly pairs Leydig cells with FSH. While Leydig cells are correct, FSH primarily targets Sertoli cells, not Leydig cells, and doesn't directly stimulate testosterone production.
Choice C incorrectly identifies Sertoli cells as testosterone producers. Sertoli cells support spermatogenesis and produce inhibin and anti-Müllerian hormone, but they don't produce significant amounts of testosterone. LH does target the correct cells (Leydig), but not Sertoli cells.
Choice D is wrong on multiple levels - Sertoli cells don't produce testosterone, and prolactin isn't the primary hormone stimulating testosterone production, though it can modulate LH sensitivity.
Remember: LH → Leydig cells → testosterone production. FSH → Sertoli cells → spermatogenesis support. This LH-Leydig-testosterone axis is frequently tested on Step 1.
- Which of the following correctly describes the vessels within a normal umbilical cord and the type of blood they carry?
Answer: Two arteries carrying deoxygenated blood, one vein carrying oxygenated blood
The umbilical cord normally contains two umbilical arteries and one umbilical vein. In a reversal of systemic circulation, the two umbilical arteries carry deoxygenated blood and waste products from the fetus to the placenta. The single, larger umbilical vein carries oxygenated and nutrient-rich blood from the placenta to the fetus.
- The relaxation of cavernosal smooth muscle leading to penile erection is primarily mediated by the local release of which of the following neurotransmitters?
Answer: Nitric oxide
Penile erection is initiated by parasympathetic nerve stimulation. These nerves release acetylcholine, which acts on endothelial cells lining the cavernosal arteries. However, the key downstream mediator is nitric oxide (NO), which is released from both non-adrenergic, non-cholinergic (NANC) nerve terminals and the endothelium. NO diffuses into the adjacent smooth muscle cells, activates guanylate cyclase, increases cGMP levels, and causes smooth muscle relaxation, leading to vasodilation, engorgement of the corpora cavernosa, and erection.
- Which component of the blastocyst is primarily responsible for invading the uterine endometrium and developing into the fetal portion of the placenta?
Answer: Trophoblast
The trophoblast is the outer layer of cells of the blastocyst. It is responsible for implantation. Upon contact with the endometrium, the trophoblast differentiates into an inner cytotrophoblast and an outer, invasive syncytiotrophoblast. The syncytiotrophoblast invades the maternal endometrium and eventually forms the chorionic villi, which are the functional units of the fetal part of the placenta.
- A newborn girl with normal-appearing external genitalia is found on imaging to lack a uterus and upper vagina. Her karyotype is 46,XX, and ovaries are present. Serum anti-Müllerian hormone (AMH) is undetectable. Which of the following developmental errors most likely produced this anatomic pattern?
Answer: Absence of Müllerian duct development despite normal ovarian differentiation
When you encounter questions about disorders of sexual development, focus on understanding which hormone systems and anatomical structures are affected versus preserved. This case presents Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome, a condition involving selective absence of Müllerian duct derivatives.
The key clues here point to complete absence of Müllerian duct development: no uterus, no upper vagina, but normal 46,XX karyotype with functioning ovaries. The undetectable AMH is actually normal for females with ovaries, since ovaries don't produce significant AMH after birth. This patient has normal ovarian development but complete failure of the Müllerian ducts to develop initially.
Answer D correctly identifies this as absence of Müllerian duct development despite normal ovarian differentiation - exactly what defines MRKH syndrome.
Answer A describes incomplete fusion of existing Müllerian ducts, which would still leave some uterine tissue present, unlike this case where the uterus is completely absent. Answer B suggests complete androgen insensitivity syndrome, but this occurs in 46,XY individuals who would have testes (not ovaries) and would have detectable AMH from testicular tissue. Answer C incorrectly focuses on Wolffian duct disruption, but the problem here is purely with Müllerian derivatives - the kidneys and other Wolffian structures are typically normal in MRKH.
Remember that MRKH syndrome specifically affects Müllerian duct formation while leaving ovarian development intact. Look for the combination of absent uterus/upper vagina with normal ovaries and 46,XX karyotype as the classic presentation.
- The development of the penis, scrotum, and prostate in a male fetus is dependent on the conversion of testosterone to which of the following hormones?
Answer: Dihydrotestosterone (DHT)
While testosterone, secreted by fetal Leydig cells, mediates the development of the internal male ductal system (Wolffian ducts), the development of the external male genitalia (penis, scrotum) and prostate requires the conversion of testosterone to the more potent androgen, dihydrotestosterone (DHT), by the enzyme 5-alpha-reductase.
- A male neonate with ambiguous external genitalia is found to have normal internal male ducts (epididymis, vas deferens, seminal vesicles) but absent prostate and hypospadias with under-virilized phallus. Karyotype is 46,XY. Testes are present and located in the inguinal canal. A defect in synthesis or action of which of the following hormones best explains the findings?
Answer: Dihydrotestosterone produced from testosterone by 5-α-reductase in peripheral tissues
When you encounter ambiguous genitalia in a 46,XY male with normal internal male structures but virilization defects, think systematically about which hormones control which aspects of male sexual development.
This patient has normal internal male ducts (epididymis, vas deferens, seminal vesicles) and absent Müllerian structures, indicating that both testosterone and anti-Müllerian hormone functioned properly during early development. However, the absent prostate, hypospadias, and under-virilized external genitalia point to a specific defect in external virilization.
The key insight is that external male genital development requires dihydrotestosterone (DHT), not just testosterone. DHT is produced when 5-α-reductase converts testosterone to this more potent androgen in peripheral tissues. DHT specifically drives development of the prostate, external genitalia, and urethral closure. A deficiency in 5-α-reductase or DHT action explains this patient's phenotype perfectly, making (A) correct.
(B) Anti-Müllerian hormone is clearly functional since Müllerian structures are absent - this hormone prevents female internal duct development. (C) Testicular testosterone is also working normally, evidenced by the presence of male internal ducts and descended testes, which require testosterone. (D) Cortisol deficiency would cause adrenal insufficiency and wouldn't selectively affect external genitalia while sparing internal male development.
Remember this pattern: normal internal male structures + abnormal external genitalia in 46,XY = think 5-α-reductase deficiency. The internal/external distinction is crucial for diagnosing disorders of sexual development.
- At the time of her birth, this woman's oocytes were arrested in which of the following stages of meiosis?
Answer: Prophase I
Oogenesis begins in fetal life. All oogonia enter meiosis I and become primary oocytes before birth. These primary oocytes are then arrested in prophase of meiosis I (specifically, the diplotene stage) and remain in this state until puberty. After puberty, a small number of oocytes resume meiosis I with each menstrual cycle.
- A 32-year-old woman notes that her basal body temperature rises by approximately 0.5 °C each month shortly after she records a peak in her urinary luteinizing hormone (LH) test. Which ovarian hormone is primarily responsible for the temperature elevation, and what is its most important source during the luteal phase?
Answer: Progesterone produced by the corpus luteum after ovulation
This question tests your understanding of the menstrual cycle's hormonal changes and their physiological effects. When you see basal body temperature changes correlated with LH surges, think about post-ovulation hormone shifts.
The temperature rise occurs because progesterone has a thermogenic effect on the hypothalamic temperature control center. After the LH surge triggers ovulation, the remnant follicle transforms into the corpus luteum, which becomes the primary source of progesterone during the luteal phase. This progesterone surge causes the characteristic 0.5°C temperature elevation that women track for fertility awareness.
Choice A is correct because progesterone from the corpus luteum is indeed responsible for the post-ovulatory temperature rise. The corpus luteum forms after ovulation and secretes large amounts of progesterone to prepare the endometrium for potential implantation.
Choice B is wrong because estradiol peaks before ovulation during the follicular phase, not after the LH surge when temperature rises. While estradiol is important for follicle development, it doesn't cause the temperature elevation.
Choice C is incorrect because inhibin B primarily regulates FSH through negative feedback and doesn't affect body temperature. Also, Sertoli cells are found in male testes, not ovaries.
Choice D is wrong because androstenedione is an androgen precursor that doesn't cause temperature changes. While theca cells do produce androgens under LH stimulation, this isn't related to the thermogenic effect.
Remember: post-ovulation temperature rise always indicates progesterone activity from the corpus luteum. This is a key fertility sign that reflects the luteal phase's hormonal dominance.
- The primary physiologic function of the corpus luteum is the secretion of which hormone to prepare the uterus for implantation?
Answer: Progesterone
The corpus luteum, formed from the remnants of the ovulated follicle under the influence of LH, is primarily responsible for producing large amounts of progesterone during the luteal (secretory) phase of the menstrual cycle. Progesterone transforms the proliferative endometrium into a secretory endometrium, which is receptive to implantation of a blastocyst.
- In a healthy fetus, which structure is primarily responsible for shunting highly oxygenated blood from the umbilical vein directly to the inferior vena cava, bypassing the hepatic circulation?
Answer: Ductus venosus
The ductus venosus is a key shunt in fetal circulation. It allows approximately half of the oxygenated blood returning from the placenta via the umbilical vein to bypass the fetal liver sinusoids and flow directly into the inferior vena cava, mixing with deoxygenated blood from the lower body before entering the right atrium.
- A 17-year-old girl with consistently irregular menses is found to have anovulatory cycles. Administration of pulsatile, rather than continuous, synthetic GnRH normalizes her menstrual pattern. The beneficial effect of the pulsatile regimen is primarily mediated by which of the following hypothalamic-pituitary-ovarian axis changes?
Answer: Restoration of cyclic LH and FSH secretion that promotes follicular development and mid-cycle LH surge
When you encounter questions about reproductive endocrinology and menstrual irregularities, focus on the hypothalamic-pituitary-ovarian axis and how disrupted GnRH pulsatility affects downstream hormone production.
This patient has anovulatory cycles that respond to pulsatile GnRH therapy, indicating a problem with normal GnRH secretion from the hypothalamus. Normally, GnRH is released in pulses every 90-120 minutes, which stimulates the anterior pituitary to secrete LH and FSH in corresponding pulses. This pulsatile pattern is crucial for normal ovarian function.
The correct answer is A because pulsatile GnRH restores the normal cyclic secretion of LH and FSH. This allows proper follicular development (FSH-dependent) and triggers the mid-cycle LH surge necessary for ovulation. Continuous GnRH administration actually downregulates GnRH receptors on pituitary gonadotrophs, suppressing LH and FSH release.
Answer B is incorrect because GnRH doesn't directly stimulate ovarian cells—it works through the pituitary to release gonadotropins. Answer C is wrong because progesterone receptor upregulation isn't the primary mechanism; the issue is lack of ovulation and progesterone production in the first place. Answer D is incorrect because this scenario isn't about prolactin suppression or dopamine—it's specifically about GnRH pulsatility.
Remember that pulsatile delivery of GnRH is therapeutic, while continuous delivery is suppressive. This principle applies to both treating hypogonadotropic hypogonadism (as in this case) and suppressing reproduction in conditions like precocious puberty or endometriosis.
- Which of the following represents a key difference between spermatogenesis and oogenesis?
Answer: Spermatogenesis produces four viable gametes, while oogenesis produces one viable gamete and polar bodies.
A primary spermatocyte undergoes meiosis I and II to produce four viable, haploid spermatids. In contrast, a primary oocyte undergoes meiosis I to produce one secondary oocyte and a small polar body. Meiosis II, if completed after fertilization, yields one large ovum and a second polar body. The unequal cytokinesis in oogenesis ensures that the ovum retains the vast majority of the cytoplasm, which is crucial for early embryonic development. The other options are incorrect: oogenesis begins in utero but is not completed, meiosis in spermatogenesis is a continuous process without prolonged arrest, and oogenesis ceases at menopause.
- A 30-year-old primigravid woman at 18 weeks' gestation is noted to have rapidly progressive maternal virilization with hirsutism and voice deepening. Ultrasonography shows a normal-appearing female fetus. Maternal serum estriol concentration is markedly decreased, and serum dehydroepiandrosterone sulfate (DHEA-S) is elevated. Placental sulfatase activity is undetectable. Which of the following best explains the low maternal estriol level?
Answer: The placenta cannot convert DHEA-S from the fetal adrenal into estriol without sulfatase activity
When you encounter a question about maternal virilization with low estriol levels during pregnancy, think about the normal pathway of estrogen synthesis and where disruption might occur. This scenario tests your understanding of fetoplacental steroidogenesis.
During normal pregnancy, the fetal adrenal glands produce large amounts of DHEA-S, which travels to the placenta. The placenta contains sulfatase enzymes that remove the sulfate group from DHEA-S, converting it to DHEA. This DHEA is then aromatized to estrone and subsequently to estriol. This pathway is crucial for maintaining normal maternal estrogen levels during pregnancy.
In this case, the absence of placental sulfatase activity creates a metabolic block. The elevated DHEA-S cannot be converted to DHEA and subsequently to estriol, explaining the markedly decreased maternal estriol levels. The accumulated DHEA-S and other androgens cause the maternal virilization symptoms. Answer A correctly identifies this mechanism.
Answer B is incorrect because the corpus luteum's role in estrogen production becomes minimal after the first trimester when placental production takes over, and LH support continues throughout pregnancy. Answer C is wrong because maternal peripheral aromatase isn't the primary source of estriol during pregnancy—the placenta is. Answer D incorrectly suggests estrogen is converted to progesterone, which doesn't occur via this pathway.
Remember: Placental sulfatase deficiency is a classic cause of low maternal estriol with maternal virilization. When you see undetectable sulfatase activity with elevated DHEA-S, think about the blocked conversion pathway from fetal adrenal precursors to maternal estrogens.
- A previously healthy 2-week-old infant is exclusively breast-fed. He shows normal growth but develops bleeding from the umbilical stump. Physical examination is otherwise unremarkable. Which of the following mechanisms best explains why parenteral vitamin K prophylaxis at birth, rather than breast milk, is necessary to prevent this complication?
Answer: Breast milk contains inadequate vitamin K because the maternal mammary gland actively limits its secretion into milk
When you encounter neonatal bleeding disorders in a healthy newborn, think about vitamin K deficiency and the unique physiology of vitamin K metabolism in neonates.
Vitamin K is essential for synthesizing clotting factors II, VII, IX, and X. Newborns are particularly vulnerable to deficiency because the maternal mammary gland actively restricts vitamin K secretion into breast milk, keeping concentrations very low. This evolutionary mechanism likely prevents excessive anticoagulation in the nursing infant, but it means breast milk alone cannot provide adequate vitamin K for proper hemostasis. Answer A correctly identifies this physiological limitation.
Answer B is incorrect because newborn intestines are essentially sterile at birth and lack the bacterial flora needed to synthesize vitamin K. The problem isn't bacterial over-utilization, but rather insufficient supply and production.
Answer C misrepresents neonatal physiology. Placental transfer of fat-soluble vitamins is actually limited, not excessive, and there's no mechanism by which this would down-regulate hepatic carboxylase enzymes involved in vitamin K-dependent protein synthesis.
Answer D is factually incorrect. Maternal estrogen doesn't competitively inhibit vitamin K absorption, and estrogen levels in breast milk aren't high enough to cause such interference even if this mechanism existed.
Study tip: Remember the "vitamin K triangle" for USMLE Step 1: newborns have (1) limited placental transfer, (2) sterile gut flora, and (3) low breast milk concentrations. This triple deficit makes parenteral vitamin K prophylaxis essential, distinguishing it from other fat-soluble vitamins.
- A 28-year-old woman undergoing in-vitro fertilization receives leuprolide as part of her controlled ovarian hyperstimulation protocol. The drug is given as a continuous subcutaneous infusion. Compared with physiologic pulsatile secretion of gonadotropin-releasing hormone (GnRH), which of the following best describes the effect of this continuous regimen on anterior pituitary gonadotropin release?
Answer: Both luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion decrease because of GnRH-receptor down-regulation on gonadotrophs
When you see questions about GnRH agonists like leuprolide, remember that the key concept is receptor regulation - continuous stimulation paradoxically leads to suppression through down-regulation.
Physiologically, GnRH is released in pulses from the hypothalamus, which maintains GnRH receptor sensitivity on anterior pituitary gonadotrophs. These pulses stimulate appropriate LH and FSH release. However, when leuprolide (a GnRH agonist) is given continuously, it initially stimulates the receptors but then causes them to become desensitized and down-regulated. This counterintuitive mechanism is why continuous GnRH agonist therapy ultimately suppresses gonadotropin release rather than stimulating it.
Answer A correctly identifies that both LH and FSH secretion decrease due to GnRH-receptor down-regulation on gonadotrophs. This suppression is precisely why leuprolide is used in IVF protocols - to prevent premature ovulation by shutting down the natural hormonal cycle before controlled stimulation begins.
Answer B incorrectly assumes continuous stimulation maintains receptor responsiveness. While there may be brief initial stimulation, sustained exposure leads to receptor desensitization. Answer C wrongly suggests selective LH suppression - both gonadotropins are affected since they're released by the same pituitary cell population responding to the same down-regulated receptors. Answer D incorrectly proposes differential receptor affinities; both LH and FSH release are controlled by the same GnRH receptors on gonadotrophs.
Remember: GnRH agonists have a biphasic effect - brief stimulation followed by prolonged suppression through receptor down-regulation. This principle applies to other continuous hormone therapies as well.
- Which of the following placental hormones is most responsible for inducing maternal insulin resistance to ensure adequate glucose delivery to the fetus?
Answer: Human placental lactogen (hPL)
Human placental lactogen (hPL), also known as human chorionic somatomammotropin, is a hormone produced by the syncytiotrophoblast. Its primary role is to stimulate maternal lipolysis and insulin resistance. This shunts carbohydrates (glucose) towards the fetus while promoting the use of free fatty acids for maternal energy, ensuring the fetus has a constant supply of nutrients.
- The surge of which of the following hormones is directly responsible for triggering ovulation within 24-36 hours?
Answer: Luteinizing hormone (LH)
The mid-cycle surge of Luteinizing Hormone (LH) from the anterior pituitary is the primary trigger for the final maturation of the dominant ovarian follicle and its rupture, a process known as ovulation. This surge is caused by a positive feedback effect of high estrogen levels from the mature follicle.
- A couple seeks genetic counseling after the birth of a daughter with congenital adrenal hyperplasia due to 21-hydroxylase deficiency. The counselor explains that female external genitalia differentiate in the absence of high androgen exposure. During which of the following gestational weeks is exposure to excess androgens most likely to produce virilization of genetically female external genitalia?
Answer: Weeks 8 to 12, when labioscrotal and urogenital folds undergo sexual differentiation
When you encounter questions about sexual differentiation, remember that external genital development follows a predictable timeline, and the window for androgen-induced virilization is narrow and specific.
External genital differentiation begins around week 8 of gestation when the initially identical external genital structures start to differentiate based on hormonal influences. The labioscrotal folds normally fuse to form the scrotum in males (under androgen influence) or remain separate to form the labia majora in females (absence of androgens). Similarly, the urogenital folds either close to form the penile raphe and ventral penis in males or remain open to form the labia minora in females. This critical period extends through week 12, making choice A correct.
Choice B is incorrect because weeks 2-4 occur before the genital tubercle even forms - there's nothing yet to virilize. Choice C (weeks 16-20) focuses on ovarian development and meiosis, but external genital differentiation is already complete by this time, so androgen exposure wouldn't cause virilization. Choice D (weeks 24-28) mentions lung maturation, which is completely unrelated to genital development and occurs far too late to influence external genital appearance.
In congenital adrenal hyperplasia with 21-hydroxylase deficiency, excess androgen production begins early in fetal development, coinciding perfectly with this 8-12 week window when external genitalia are actively differentiating.
Remember: External genital virilization can only occur during active differentiation (weeks 8-12). Before this window, structures don't exist; after this window, differentiation is complete and irreversible through hormonal exposure alone.
- A 24-year-old man develops bilateral testicular atrophy after several months of self-administered anabolic-androgenic steroids. Serum testosterone is markedly elevated, and sperm count is near zero. Which of the following intratesticular changes is most directly responsible for the severe oligospermia?
Answer: Decreased Sertoli-cell production of inhibin B and androgen-binding protein in response to suppressed FSH
When you encounter questions about anabolic steroid abuse and testicular dysfunction, focus on the hypothalamic-pituitary-gonadal axis and how exogenous hormones disrupt normal feedback mechanisms.
Exogenous anabolic steroids provide supraphysiologic testosterone levels that create negative feedback inhibition at the hypothalamus and anterior pituitary. This suppresses both LH and FSH secretion. While the patient has elevated serum testosterone from the steroids, his intratesticular testosterone production is actually decreased due to LH suppression. More critically for spermatogenesis, FSH suppression dramatically reduces Sertoli cell function.
Sertoli cells require FSH stimulation to produce inhibin B and androgen-binding protein (ABP), both essential for spermatogenesis. ABP concentrates testosterone locally in the seminiferous tubules at levels much higher than systemic circulation can provide. Without adequate FSH stimulation, Sertoli cells cannot maintain this crucial microenvironment, leading to severe oligospermia despite elevated systemic testosterone. This makes option A correct.
Option B is incorrect because while Leydig cells are suppressed by LH inhibition, the primary cause of oligospermia is Sertoli cell dysfunction, not Leydig cell death. Option C describes a congenital abnormality (patent processus vaginalis) unrelated to steroid use. Option D mentions aromatization to estradiol, which does occur but isn't the direct intratesticular mechanism causing oligospermia.
Remember: spermatogenesis depends more on local intratesticular factors (FSH-stimulated Sertoli cell products) than systemic hormone levels. Exogenous steroids paradoxically cause infertility despite high testosterone levels.
