Reproductive Physiology — MCQs

Reproductive Physiology Indian Medical PG Practice Questions and MCQs

Question 31: Which enzyme is the rate-limiting step in testosterone synthesis?

A. 17,20 lyase
B. 20,22-desmolase (Correct Answer)
C. 3-beta-hydroxysteroid dehydrogenase (3-beta-HSD)
D. 17-hydroxylase

Explanation: **Explanation:** **Correct Answer: B. 20,22-desmolase** The synthesis of all steroid hormones, including testosterone, begins with cholesterol. The conversion of **cholesterol to pregnenolone** is the first and **rate-limiting step** in steroidogenesis. This reaction is catalyzed by the enzyme **20,22-desmolase** (also known as Cholesterol Side-Chain Cleavage enzyme or **CYP11A1**), located on the inner mitochondrial membrane. In the testes, this step is primarily regulated by **Luteinizing Hormone (LH)**, which increases the transport of cholesterol into the mitochondria via the **StAR (Steroidogenic Acute Regulatory) protein**. **Analysis of Incorrect Options:** * **A. 17,20 lyase:** This enzyme converts 17-hydroxypregnenolone to dehydroepiandrosterone (DHEA). While essential for the androgenic pathway, it is not the rate-limiting step. * **C. 3-beta-HSD:** This enzyme converts pregnenolone to progesterone (and DHEA to androstenedione). It is a crucial step in the synthesis of all classes of steroid hormones but is not the bottleneck of the pathway. * **D. 17-hydroxylase:** This enzyme converts pregnenolone to 17-hydroxypregnenolone. While it "shunts" precursors toward the sex steroid/cortisol pathways (away from aldosterone), it is not the overall rate-limiting enzyme. **High-Yield Clinical Pearls for NEET-PG:** * **StAR Protein:** The actual "bottleneck" at the molecular level is the transport of cholesterol into the mitochondria by the StAR protein. * **Location:** 20,22-desmolase is found in the **Leydig cells** of the testes. * **Final Step:** The final step in testosterone synthesis is the conversion of androstenedione to testosterone by **17β-hydroxysteroid dehydrogenase (17β-HSD)**. * **Potency:** Testosterone is converted to the more potent **Dihydrotestosterone (DHT)** by the enzyme **5-alpha-reductase** in peripheral tissues.

Question 32: The first meiotic division of oogenesis gets arrested at which stage?

A. Pachytene stage of prophase
B. Diplotene stage of prophase (Correct Answer)
C. Leptotene stage of prophase
D. Metaphase stage of prophase

Explanation: **Explanation:** In females, oogenesis begins during fetal life. Primary oocytes initiate the first meiotic division (Meiosis I) but do not complete it until puberty. **Why Diplotene is correct:** The first meiotic arrest occurs during the **Diplotene stage of Prophase I**. This specific period of arrest is also known as the **Dictyate stage**. It is mediated by Oocyte Maturation Inhibitor (OMI) secreted by follicular cells. The oocyte remains in this suspended state for years—from birth until just before ovulation—at which point the LH surge triggers the completion of Meiosis I. **Analysis of Incorrect Options:** * **Leptotene & Pachytene:** These are earlier stages of Prophase I. While DNA condensation and homologous recombination occur here, the cell does not arrest at these points. * **Metaphase:** This is incorrect for the *first* arrest. However, it is a high-yield distractor because the **second meiotic arrest** occurs at **Metaphase II**. This second arrest is only released if fertilization by a sperm occurs. **NEET-PG High-Yield Pearls:** * **First Arrest:** Prophase I (Diplotene/Dictyate stage) — occurs at birth. * **Second Arrest:** Metaphase II — occurs at ovulation; completed only upon fertilization. * **Meiosis I Completion:** Results in the formation of a secondary oocyte and the **first polar body**. * **Clinical Correlation:** The long duration of the diplotene arrest (up to 40-50 years) is a primary reason for the increased risk of chromosomal non-disjunction (e.g., Trisomy 21) in older maternal ages.

Question 33: What hormone controls the mucosal lining of the fallopian tube secretion for ovum nutrition?

A. LH
B. FSH
C. Estrogen
D. Progesterone (Correct Answer)

Explanation: **Explanation:** The correct answer is **Progesterone**. The fallopian tube (oviduct) contains two main types of epithelial cells: ciliated cells and non-ciliated **peg cells**. While estrogen is responsible for the proliferation of the mucosa and increasing the number of cilia, **progesterone** is the primary hormone that stimulates the secretory activity of the peg cells. These secretions provide a nutrient-rich environment (containing glycogen, electrolytes, and amino acids) essential for the survival of the ovum and the nourishment of the zygote during its transit through the tube. **Why other options are incorrect:** * **LH (Luteinizing Hormone):** Its primary role is to trigger ovulation and maintain the corpus luteum; it does not directly regulate tubal secretions. * **FSH (Follicle Stimulating Hormone):** It stimulates the growth of ovarian follicles and the production of estrogen but has no direct effect on the fallopian tube mucosa. * **Estrogen:** Estrogen promotes the **ciliogenesis** (formation and activity of cilia) and increases the vascularity of the tubes. While it prepares the tube, it is progesterone that triggers the actual "secretory phase" of the tubal lining. **High-Yield NEET-PG Pearls:** * **Ciliary movement:** Estrogen increases the frequency of ciliary beating to facilitate gamete transport. * **Peg Cells:** These are non-ciliated, secretory cells that bulge into the lumen; their activity peaks in the post-ovulatory (luteal) phase under progesterone influence. * **Capacitation:** The secretions stimulated by progesterone also play a role in the final maturation of sperm (capacitation) within the female reproductive tract.

Question 34: Which of the following parameters is decreased during the third trimester of a normal pregnancy?

A. Heart rate
B. Stroke volume
C. Systemic vascular resistance (Correct Answer)
D. Cardiac output

Explanation: **Explanation:** The correct answer is **Systemic Vascular Resistance (SVR)**. During pregnancy, the cardiovascular system undergoes significant remodeling to accommodate the metabolic demands of the fetus. **1. Why Systemic Vascular Resistance (SVR) decreases:** SVR begins to fall as early as the first trimester, reaching its nadir (lowest point) during the second and third trimesters (a 20–30% reduction). This occurs due to two primary mechanisms: * **Progesterone and Nitric Oxide:** High levels of progesterone and increased synthesis of nitric oxide and prostaglandins cause significant peripheral vasodilation. * **Uteroplacental Circulation:** The placenta acts as a large, **low-resistance shunt** added in parallel to the maternal circulation, further lowering the overall resistance. **2. Why the other options are incorrect:** * **Cardiac Output (CO):** CO **increases** by 30–50% during pregnancy. It peaks by the end of the second trimester and remains elevated through the third. * **Heart Rate (HR):** HR gradually **increases** throughout pregnancy, typically rising by 10–20 beats per minute above baseline. * **Stroke Volume (SV):** SV **increases** significantly, especially in early pregnancy, due to increased plasma volume and improved myocardial contractility. **High-Yield Facts for NEET-PG:** * **Blood Pressure:** Despite the increase in CO, Mean Arterial Pressure (MAP) **decreases** slightly because the drop in SVR is more profound. * **Plasma Volume:** Increases by ~50%, while RBC mass increases by ~20-30%, leading to **Physiological Anemia of Pregnancy**. * **Supine Hypotension Syndrome:** In the third trimester, the gravid uterus can compress the Inferior Vena Cava (IVC) when the mother lies supine, reducing venous return and CO. Always advise the **left lateral position**.

Question 35: Most of the testosterone secreted by the testes exists in the plasma in the form of?

A. Dihydrotestosterone bound to gonadal steroid-binding hormone
B. Free dihydrotestosterone
C. Free testosterone
D. Testosterone bound to sex-steroid-binding globulin (Correct Answer)

Explanation: ### Explanation **1. Why the Correct Answer is Right:** Testosterone is a lipophilic steroid hormone that requires carrier proteins for transport in the aqueous environment of the plasma. In adult males, approximately **98% of circulating testosterone is protein-bound**. The majority (about **65-70%**) is bound with high affinity to **Sex-Steroid-Binding Globulin (SSBG)**, also known as Sex Hormone-Binding Globulin (SHBG). The remaining bound fraction (approx. 30%) is loosely attached to **Albumin**. Only the unbound or "free" fraction (approx. 2%) is biologically active and capable of entering target cells. **2. Why the Incorrect Options are Wrong:** * **Options A & B (Dihydrotestosterone):** While Dihydrotestosterone (DHT) is a more potent androgen, it is primarily formed in peripheral tissues (like the prostate and skin) by the action of the enzyme 5-alpha-reductase on testosterone. It is not the primary form in which testosterone itself exists in the plasma. * **Option C (Free Testosterone):** Only a very small fraction (about **2%**) of testosterone exists in the free, unbound state. While this fraction is physiologically the most important for hormonal action, it does not represent the "most" common form in the plasma. **3. High-Yield Clinical Pearls for NEET-PG:** * **Bioavailable Testosterone:** This refers to the sum of **Free Testosterone + Albumin-bound Testosterone**. Because the binding to albumin is weak/low-affinity, it dissociates easily for tissue uptake. * **SHBG Regulation:** SHBG levels are **increased** by Estrogen and Hyperthyroidism, and **decreased** by Androgens, Obesity, and Nephrotic syndrome. * **Origin:** Testosterone is secreted by the **Leydig cells** of the testes under the influence of **Luteinizing Hormone (LH)**.

Question 36: All of the following physiological changes occur in pregnancy EXCEPT?

A. Increase in cardiac output
B. Increase in stroke volume
C. Decrease in fibrinogen levels (Correct Answer)
D. Rise in the venous pressure in the lower limb in the supine position

Explanation: **Explanation:** Pregnancy is a state of significant physiological adaptation across multiple organ systems to support the growing fetus. **Why Option C is the Correct Answer:** Pregnancy is a **hypercoagulable state**. There is a significant **increase** (not decrease) in the levels of most clotting factors, particularly **Fibrinogen (Factor I)**, which can rise by up to 50% (reaching 400–600 mg/dL). This serves as a protective mechanism to prevent postpartum hemorrhage. Other changes include a decrease in Protein S activity and an increase in Plasminogen Activator Inhibitor (PAI-1 and PAI-2), further increasing the risk of thromboembolism. **Analysis of Incorrect Options:** * **Option A & B:** Cardiac output (CO) increases by 30–50% during pregnancy. This is achieved by an **increase in Stroke Volume (SV)** (predominantly in the first half of pregnancy) and an **increase in Heart Rate (HR)** (predominantly in the second half). Since $CO = SV \times HR$, both options are physiological features of pregnancy. * **Option D:** In the supine position, the gravid uterus compresses the **Inferior Vena Cava (IVC)**. This mechanical obstruction leads to increased venous pressure in the lower limbs, often resulting in dependent edema and varicose veins. It can also lead to "Supine Hypotension Syndrome" due to decreased venous return. **High-Yield NEET-PG Pearls:** * **Hematology:** Plasma volume increases (50%) more than RBC mass (20–30%), leading to **physiological anemia**. * **Respiratory:** Tidal Volume increases (Progesterone effect), but **Functional Residual Capacity (FRC) decreases** due to the elevation of the diaphragm. * **Renal:** GFR and Renal Plasma Flow increase, leading to a **decrease in serum Creatinine and Urea** levels.

Question 37: What are the typical prolactin levels during pregnancy?

A. Lowest during pregnancy
B. Highest during pregnancy (Correct Answer)
C. Unaffected by pregnancy and lactation
D. Variable in every pregnancy

Explanation: **Explanation:** **Why Option B is Correct:** During pregnancy, prolactin levels undergo a progressive and significant increase, reaching their peak (highest levels) during the third trimester. This rise is primarily driven by high levels of **estrogen** secreted by the placenta. Estrogen stimulates the lactotrophs in the anterior pituitary gland to undergo both hyperplasia (increase in cell number) and hypertrophy (increase in cell size). By the end of pregnancy, prolactin levels can be 10 to 20 times higher than non-pregnant levels (reaching up to 200 ng/mL), preparing the mammary glands for lactation. **Why Other Options are Incorrect:** * **Option A:** Prolactin is never at its lowest during pregnancy; in fact, it is one of the hormones that shows the most dramatic increase. * **Option C:** Prolactin is profoundly affected by both. Pregnancy increases its baseline level, while lactation (specifically the suckling reflex) causes episodic surges to maintain milk production. * **Option D:** While exact numerical values may vary slightly between individuals, the physiological trend of a steady, significant increase is a universal feature of normal human pregnancy. **High-Yield Clinical Pearls for NEET-PG:** * **The Estrogen Paradox:** Although prolactin levels are highest during pregnancy, milk secretion (lactation) does not occur *in utero*. This is because high levels of **estrogen and progesterone** competitively inhibit the action of prolactin on the breast tissue. Lactation only begins postpartum once these steroid levels plummet. * **Pituitary Size:** Due to lactotroph hyperplasia, the pituitary gland increases in size by nearly 50-100% during pregnancy, making it more susceptible to infarction if massive hemorrhage occurs (**Sheehan’s Syndrome**). * **Amniotic Fluid:** Prolactin is also found in high concentrations in amniotic fluid, where it helps regulate fetal osmoregulation.

Question 38: On which day of the menstrual cycle does endometrium regeneration start?

A. 5th day (Correct Answer)
B. 7th day
C. 14th day
D. 28th day

Explanation: **Explanation:** The menstrual cycle is divided into the menstrual, proliferative, and secretory phases. The **proliferative phase** (also known as the follicular or estrogenic phase) is responsible for the regeneration of the endometrium. 1. **Why Option A is correct:** Menstruation typically lasts from day 1 to day 4 or 5. As the levels of estrogen begin to rise (secreted by the developing secondary follicles), the **stratum basalis** starts to proliferate to reconstruct the **stratum functionalis** which was shed. This regenerative process begins immediately as the menstrual flow ceases, which is typically around the **5th day** of the cycle. 2. **Why Option B is incorrect:** By the 7th day, the endometrium is already well into the proliferative phase; regeneration has already been established. 3. **Why Option C is incorrect:** The 14th day marks **ovulation**. At this point, the proliferative phase ends, and the endometrium transitions into the secretory phase under the influence of progesterone. 4. **Why Option D is incorrect:** The 28th day marks the end of the secretory phase. If fertilization does not occur, the withdrawal of progesterone leads to tissue necrosis and the start of a new menstrual bleed. **NEET-PG High-Yield Pearls:** * **Regenerative Layer:** The *Stratum Basalis* is the permanent layer that does not shed; it contains the stem cells responsible for regeneration. * **Hormonal Control:** Estrogen is the primary hormone for the proliferative phase, while Progesterone dominates the secretory phase. * **Thickness:** The endometrium grows from ~1 mm to about 3–5 mm during the proliferative phase. * **Histology:** Look for "straight glands" and "mitotic figures" in the proliferative phase, versus "tortuous/corkscrew glands" in the secretory phase.

Question 39: From where was prostaglandin discovered?

A. Tear
B. Saliva
C. Seminal fluid (Correct Answer)
D. Blood

Explanation: **Explanation:** Prostaglandins were first discovered in **seminal fluid** in the 1930s. The discovery is attributed to **Ulf von Euler**, who isolated these lipid compounds from human semen. At the time, it was mistakenly believed that these substances were secreted exclusively by the **prostate gland** (hence the name "prostaglandin"). However, it was later discovered that the majority of prostaglandins in semen are actually produced by the **seminal vesicles**. **Analysis of Options:** * **Option C (Correct):** Seminal fluid contains the highest concentration of prostaglandins in the body. Their physiological role in semen is to stimulate contractions of the female reproductive tract (uterus and fallopian tubes) to facilitate sperm transport toward the ovum. * **Options A, B, and D (Incorrect):** While prostaglandins are now known to be ubiquitous paracrine hormones produced by almost all nucleated cells in the body (found in blood, saliva, and tears), they were not the original source of discovery. Their concentrations in these fluids are significantly lower than in seminal fluid. **High-Yield Clinical Pearls for NEET-PG:** * **Precursor:** Prostaglandins are derived from **Arachidonic acid** (a 20-carbon polyunsaturated fatty acid) via the **Cyclooxygenase (COX) pathway**. * **Mechanism:** They act as local hormones (autocrine/paracrine) via G-protein coupled receptors. * **Clinical Use:** Prostaglandin E2 (Dinoprostone) is used for cervical ripening and induction of labor; PGE1 (Alprostadil) is used to keep the Ductus Arteriosus patent in cyanotic heart disease. * **Inhibition:** NSAIDs (like Aspirin) exert their anti-inflammatory and analgesic effects by inhibiting the COX enzyme, thereby reducing prostaglandin synthesis.

Question 40: Quiescence of the uterine musculature is:

A. Enhanced by progesterone (Correct Answer)
B. Enhanced by estrogen
C. Inhibited by progesterone
D. Enhanced by estrogen and inhibited by progesterone

Explanation: **Explanation:** The term **quiescence** refers to the state of relative inactivity or "quietness" of the uterine musculature (myometrium) during pregnancy. This state is essential to prevent premature labor and allow the fetus to reach full term. **1. Why Option A is Correct:** Progesterone is the primary hormone responsible for maintaining uterine quiescence. It exerts a "progesterone block" through several mechanisms: * **Hyperpolarization:** It increases the resting membrane potential of myometrial cells, making them less excitable. * **Inhibition of Gap Junctions:** It decreases the formation of gap junctions (connexin-43), preventing the coordinated spread of electrical signals. * **Downregulation of Receptors:** It decreases the expression of oxytocin receptors and prostaglandin receptors, which are essential for contractions. **2. Why the Other Options are Incorrect:** * **Option B:** **Estrogen** does the opposite; it acts as a "pro-contractile" hormone. It increases the synthesis of actin and myosin, promotes gap junction formation, and upregulates oxytocin receptors, thereby increasing uterine excitability. * **Option C:** Progesterone **enhances** quiescence; it does not inhibit it. * **Option D:** This is incorrect because estrogen promotes contractility (inhibits quiescence), while progesterone promotes quiescence. **High-Yield Clinical Pearls for NEET-PG:** * **Progesterone Withdrawal:** Toward the end of pregnancy, the functional withdrawal of progesterone (or a decrease in the progesterone/estrogen ratio) is a key trigger for the onset of labor. * **Clinical Use:** Progesterone supplementation (e.g., hydroxyprogesterone caproate) is clinically used to prevent **preterm labor** in high-risk women by maintaining this quiescence. * **Ferguson Reflex:** This is the neuroendocrine reflex where cervical stretching leads to oxytocin release, overriding quiescence to facilitate labor.

Practice by Chapter

Male Reproductive Physiology

Practice Questions

Spermatogenesis and Sperm Function

Practice Questions

Female Reproductive Physiology

Practice Questions

Menstrual Cycle

Practice Questions

Ovulation and Fertilization

Practice Questions

Physiology of Pregnancy

Practice Questions

Parturition

Practice Questions

Lactation

Practice Questions

Sexual Differentiation and Development

Practice Questions

Reproductive Aging

Practice Questions

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