8Reproductive system and development w solutions

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BIOLOGY TOPICAL: Reproductive System and Development Test Time: 21 Minutes* Number of Questions: 16 * The timing restrictions for the science topical tests are optional If you are using this test for the sole purpose of content reinforcement, you may want to disregard the time limit MCAT DIRECTIONS: Most of the questions in the following test are organized into groups, with a descriptive passage preceding each group of questions Study the passage, then select the single best answer to each question in the group Some of the questions are not based on a descriptive passage; you must also select the best answer to these questions If you are unsure of the best answer, eliminate the choices that you know are incorrect, then select an answer from the choices that remain Indicate your selection by blackening the corresponding circle on your answer sheet A periodic table is provided below for your use with the questions PERIODIC TABLE OF THE ELEMENTS H 1.0 He 4.0 Li 6.9 Be 9.0 B 10.8 C 12.0 N 14.0 O 16.0 F 19.0 10 Ne 20.2 11 Na 23.0 12 Mg 24.3 13 Al 27.0 14 Si 28.1 15 P 31.0 16 S 32.1 17 Cl 35.5 18 Ar 39.9 19 K 39.1 20 Ca 40.1 21 Sc 45.0 22 Ti 47.9 23 V 50.9 24 Cr 52.0 25 Mn 54.9 26 Fe 55.8 27 Co 58.9 28 Ni 58.7 29 Cu 63.5 30 Zn 65.4 31 Ga 69.7 32 Ge 72.6 33 As 74.9 34 Se 79.0 35 Br 79.9 36 Kr 83.8 37 Rb 85.5 38 Sr 87.6 39 Y 88.9 40 Zr 91.2 41 Nb 92.9 42 Mo 95.9 43 Tc (98) 44 Ru 101.1 45 Rh 102.9 46 Pd 106.4 47 Ag 107.9 48 Cd 112.4 49 In 114.8 50 Sn 118.7 51 Sb 121.8 52 Te 127.6 53 I 126.9 54 Xe 131.3 55 Cs 132.9 56 Ba 137.3 57 La * 138.9 72 Hf 178.5 73 Ta 180.9 74 W 183.9 75 Re 186.2 76 Os 190.2 77 Ir 192.2 78 Pt 195.1 79 Au 197.0 80 Hg 200.6 81 Tl 204.4 82 Pb 207.2 83 Bi 209.0 84 Po (209) 85 At (210) 86 Rn (222) 87 Fr (223) 88 Ra 226.0 89 Ac † 227.0 104 Unq (261) 105 Unp (262) 106 Unh (263) 107 Uns (262) 108 Uno (265) 109 Une (267) * 58 Ce 140.1 59 Pr 140.9 60 Nd 144.2 61 Pm (145) 62 Sm 150.4 63 Eu 152.0 64 Gd 157.3 65 Tb 158.9 66 Dy 162.5 67 Ho 164.9 68 Er 167.3 69 Tm 168.9 70 Yb 173.0 71 Lu 175.0 † 90 Th 232.0 91 Pa (231) 92 U 238.0 93 Np (237) 94 Pu (244) 95 Am (243) 96 Cm (247) 97 Bk (247) 98 Cf (251) 99 Es (252) 100 Fm (257) 101 Md (258) 102 No (259) 103 Lr (260) GO ON TO THE NEXT PAGE as developed by Reproductive System and Development Test Passage I (Questions 1–5) The process of sex differentiation involves the development of male or female characteristics, including differentiation of the gonads, the internal reproductive tract, and the external genitalia In mammals, the male genotype is XY and the female genotype is XX It is the Y chromosome that acts as the male determinant In its absence, development follows the female pathway The first sign of male differentiation is the appearance of Sertoli cells in the 7th week of pregnancy Sertoli cells are thought to influence the differentiation of other cell types in the male reproductive system Therefore, the riddle of sex determination can largely be reduced to the trigger that results in the appearance of Sertoli cells Recent studies indicate that the SRY gene, located on the Y chromosome, is expressed in Sertoli cell precursors, directing them toward the Sertoli-cell pathway away from the default follicle-cell pathway Under the influence of the SRY gene, the fetal testis develops In the absence of the SRY gene, the ovaries develop Subsequent differences between the sexes are largely due to Müllerian-inhibiting substance (MIS) and testosterone, both synthesized by the fetal testis MIS is responsible for the regression of the Müllerian ducts, while testosterone promotes the development of the Wolffian duct system into the male reproductive tract (vas deferens, seminal vesicles, etc.) In females, the absence of MIS and testosterone results in the formation of the female reproductive tract (oviducts, uterus, and vagina) In genotypic males, if the fetal testis fails to produce MIS or testosterone, the female reproductive structures develop At puberty, testosterone directs the development of male external genitalia and secondary sex characteristics External sex characteristics are masculinized in the presence of testosterone and feminized in its absence Which of the following evidence best supports the theory that SRY is the male-determining gene? A The discovery of an SRY homolog on the X chromosome B The SRY gene is activated in the fetus during the first trimester of pregnancy C The SRY gene is expressed only in male sex cells D The existence of an XY individual who has feminized external genitalia One type of male pseudohermaphrodite is an XY individual with both testes and female external genitalia, due to the absence of testosterone Which of following is most likely true of such an individual? A Neither Wolffian nor Müllerian ducts are present B The sex cells are diploid, carrying both X and Y chromosomes C The SRY gene is not functional D A male pseudohermaphrodite is capable of selffertilization Based on information in the passage, it can be inferred that an XO individual: A cannot survive B will have immature, ambiguous (both male and female) reproductive structures C will be phenotypically female D does not produce steroid hormones GO ON TO THE NEXT PAGE KAPLAN MCAT Testosterone probably promotes development of the Wolffian structures into the male reproductive tract by: A binding to testosterone receptors on the surface of its target cells B effecting cellular changes via a second messenger, such as cAMP C entering the nuclei of its target cells and activating specific genes D inhibiting Müllerian duct developments thereby indirectly promoting the development of male structures What is the probability that the second child produced by a genotypically and phenotypically normal couple will be female, if their first child was male? A B C D 25% 50% 75% 100% GO ON TO THE NEXT PAGE as developed by Reproductive System and Development Test Passage II (Questions 6–11) Endometriosis, a common cause of female sterility, is a condition in which endometrial tissue grows in locations outside the uterus, typically within the pelvic cavity This abnormal endometrial tissue is nearly identical to that of the uterine endometrium, and is under the influence of the same ovarian hormones Therefore, abnormal endometrial tissue periodically menstruates This leads to an extensive accumulation of blood that causes the development of fibrous tissue (fibrosis) throughout the pelvis Fibrosis can enshroud the ovaries as well as block the fallopian tubes There are three theories as to the cause of endometriosis The first, known as the regurgitation theory, postulates that endometriosis is the result of backflow of menstrual tissue through the fallopian tubes The second, the metaplastic theory, proposes that coelomic epithelium (the lining of the abdominal cavity) is transformed into abnormal endometrial tissue The third theory, known as the vascular or lymphatic dissemination theory, suggests that endometrial glands are transported from the uterus to other locations by the circulatory or lymphatic systems Women suffering from endometriosis usually experience severe pelvic pain and painful menstruation Treatment depends on the age and health of the patient, as well as her desire to have children One possible treatment is the pharmacological suppression of ovarian hormone function to arrest the activity of the abnormal tissue Another course of action is the surgical removal of as much of the abnormal tissue as possible Suppose that a woman with endometriosis takes estrogen and progesterone pills continually to suppress ovarian hormone function One event of the reproductive cycle that will nonetheless occur is: A B C D secretion of gonadotropin-releasing hormone implantation of the ovum atrophy of the corpus luteum thickening of the endometrium Endometrial tissue has been found in sites distant from the pelvic cavity, including the lungs and the nasal mucosa Which of the theories is consistent with this information? I Regurgitation theory II Metaplastic theory III Vascular or lymphatic dissemination theory A B C D I only III only I and III only II and III only In 25–50% of cases of female infertility, endometriosis is diagnosed as the underlying cause The most likely reason that a woman with endometriosis becomes infertile is that: The periodic bleeding of the non-uterine endometrial tissue is directly induced by which of the following hormonal changes? A B C D Increased secretion of FSH and LH Decreased secretion of FSH and LH Increased secretion of estrogen and progesterone Decreased secretion of estrogen and progesterone A the ovum cannot enter the fallopian tubes B the zygote becomes implanted in abnormal endometrial tissue C ovarian hormone function is suppressed D ovulation cannot occur GO ON TO THE NEXT PAGE KAPLAN MCAT It is known that retrograde menstruation through the fallopian tubes is common even in healthy women It can therefore be inferred that if the regurgitation theory is correct, then: A endometriosis is probably the result of differentiation of the coelomic epithelial lining B genetic, hormonal, or immunological factors are probably also involved in endometriosis C there must be a variety of conditions that cause irregular menstruation D endometriosis is a natural occurrence in the course of most women’s reproductive lives 1 In patients with endometriosis, the ovaries may become distorted by cystic spaces filled with brown blood debris Surgical removal of the ovaries would: A elicit the continuous production of LH and FSH B have little effect on endometrial tissue outside the uterus C only be possible during the second half of the patient’s monthly cycle D cause the production of all hormones in the reproductive cycle to be halted GO ON TO THE NEXT PAGE as developed by Reproductive System and Development Test Questions 12 through 16 are NOT based on a descriptive passage Smooth muscle develops from which of the following germ layers? A B C D Endoderm Mesoderm Epiderm Ectoderm In a chick embryo, some ectodermal cells develop into wing feathers, while others develop into thigh feathers or claws of the feet A researcher transplants cells from an area of the ectoderm that normally develops into wing feathers to an area of the ectoderm that normally develops into claws of the feet The transplanted cells subsequently developed into claws because: A B C D the fate of the cells had already been determined ectodermal cells possess positional information the cells were induced by underlying mesoderm ectodermal cells can develop into any type of tissue When a sperm cell penetrates the corona radiata and the zona pellucida of a secondary oocyte in the process of fertilization, the nucleus of the oocyte divides into a mature ovum and a polar body Which of the following indicates that this division is part of meiosis II, NOT meiosis I or mitosis? A The division is not preceded by DNA replication B There are 23 pairs of chromosomes in the secondary oocyte prior to division C The polar body contains the haploid number of chromosomes D Each chromosome in the mature ovum consists of two sister chromatids In the placenta, fetal blood has a P O of only 30 mmHg while maternal blood has a PO2 of 50 mmHg, yet fetal blood can transport nearly as much oxygen to the fetal tissues as maternal blood can transport to the maternal tissues Which of the following does NOT account for this phenomenon? A The concentration of fetal hemoglobin in fetal blood is greater than the concentration of adult hemoglobin in adult blood B Fetal hemoglobin has a greater affinity for oxygen than does maternal hemoglobin for a given PO2 C The newborn begins to produce adult hemoglobin after birth D The capacity of both fetal and adult hemoglobin to carry oxygen decreases as PCO2 increases All of the following are functions of the placenta EXCEPT: A secretion of human chorionic gonadotropin B active absorption of nutrients from the mother’s blood C excretion of carbon dioxide into the mother’s blood D protection of the fetus from drugs and toxins END OF TEST KAPLAN MCAT ANSWER KEY: B A C C B 10 D D B A B 11 12 13 14 15 A B C D A 16 C as developed by Reproductive System and Development Test REPRODUCTIVE SYSTEM AND DEVELOPMENT TEST TRANSCRIPT Passage (Questions 1-5) The correct answer is choice B Step back a second and think about what you know In order for the SRY gene to be the male-determining gene, it must be expressed only in males, and it must be expressed in the right cells, at the right time Choice A suggests that the discovery of an SRY homolog on the X chromosome would support the role of the SRY gene as the male-determinant The term "homolog" refers to a gene on the X chromosome that corresponds to the SRY gene on the Y chromosome The existence of such a homolog would mean that the SRY gene is not specific to males, and is therefore most likely not the male-determining gene Thus, choice A would not support the SRY gene theory On the other hand, determining that the SRY gene is activated during the first trimester of pregnancy (weeks through 12) would support the SRY gene theory You're told in the second paragraph that the first sign of differentiation is the appearance of Sertoli cells in the 7th week of pregnancy Therefore, the male-determining gene must be activated prior to this event, and so choice B fits this timeline Since the question asks for the piece of evidence which best supports the theory, we need to check the other choices to see if one provides stronger evidence According to the passage, the fetal-testis develops normally under the influence of the SRY gene, which is expressed in the precursors of Sertoli cells, which are autosomal, and hence diploid, cells Finding that the SRY gene was expressed only in the male sex cells, a.k.a sperm, as in choice C, would most contradict the theory proposed in the passage So, choice C is wrong I'm sure you were tempted by choice C it sounds like a logical choice But remember, sperm are not produced until puberty! Choice D presents an interesting situation: an XY individual, that is, a genotypic male, who has feminized external genitalia despite the presence of the Y chromosome An individual who carries the SRY gene and yet does not develop masculine external genitalia neither supports nor weakens the SRY gene theory What most likely happened is that the fetal testis in this XY individual was unable to produce testosterone and therefore the differentiation of the external genitalia followed the female pathway during development So, choice B is the correct answer The correct answer is choice A A pseudohermaphrodite is an individual with the genetic makeup and gonads of one sex and the genitalia of the other The fetal testis normally produces two developmentally important hormones, testosterone and MIS The testosterone causes the Wolffian ducts to develop and the MIS causes the Müllerian ducts to regress The question stem tells you that the testes developed but that no testosterone is produced You can assume then that the fetal testis did produce MIS, because you're not told otherwise So, the MIS, which is produced, causes the Müllerian ducts to regress, while the Wolffian ducts, in the absence of testosterone, fail to develop Thus, choice A is correct Choice B is wrong because an individual's sex cells are not key players in his or her own embryonic sexual differentiation Sex cells are the haploid cells produced during gametogenesis Gametes are the sperm and ova that might someday unite to form a zygote Diploid sex cells carrying both the X and Y genes can be formed as the result of a nondisjunction event during meiosis So though it is possible for a sperm cell to carry both sex genes, a male's sperm cells, normal or abnormal, not influence embryonic development in any way, especially since sperm is not formed until puberty Choice C says that a nonfunctional SRY gene is the culprit, yet you know that this can't be true because he has testes So, choice C is wrong As for choice D: a pseudohermaphrodite is not a true hermaphrodite Pseudohermaphrodites not have both testes and ovaries Besides the fact that mammals are not naturally capable of self-fertilization, the process would require both sperm and an ovum, as well as a fully functioning penis, vagina, uterus, ovaries and testes Clearly, choice D is not even within the realm of the possible Again, choice A is the correct answer Choice C is correct For this question, outside knowledge would have served you well but was not required You might be familiar with Turner's syndrome A Turner's female has the genotype XO She carries only one X and does not mature sexually, but she is clearly female Turner's patients have what is known as "streak gonads," or undeveloped gonads These individuals are often shorter than normal and may have varying degrees of mental development problems So, choice C is the right answer: XO individuals are phenotypically female But you didn't need this outside knowledge to answer this question An XO individual does not have the Y chromosome and will therefore follow the female differentiation pathway of development Choice A is an extreme answer choice never say never unless the passage clearly warrants such a definitive statement, which it does not, in this case As a rule on the MCAT, stay away from answer choices that include "absolutes" such as always, never, and cannot, etc As we have just discussed, XO individuals survive It is interesting to note though, that experts in human genetics believe that XO embryos are more common than previously suspected, but most of these embryos die early in development Choice B asserts that an XO individual will have both male and female reproductive structures, but as we are told in the passage, without the Y chromosome, and specifically, without the SRY gene, there will be no testes development, and thus no male differentiation So, choice B is wrong Choice D is also incorrect because the sex steroids are not the only kind of steroid hormones, and the gonads are not the only site of steroid synthesis Other steroid hormones include aldosterone and cortisol of the corticoid family, and the members of the progestogen family, like progesterone The adrenal gland synthesizes androgens as well as the corticoid hormones Again, choice C is the right answer The correct answer is choice C Question is actually an outside knowledge question, requiring you to recall information about the mechanisms of steroid hormone action Recall that steroid hormones are relatively small, hydrophobic KAPLAN MCAT molecules and pass through cell membranes by diffusion Steroids bind their receptor proteins inside the cell to form steroidreceptor complexes The binding of the hormone changes the receptor's conformation and increases the receptor's affinity for DNA This complex is now considered "activated" and is able to bind to specific genes in the nucleus and regulate their transcription Choice C, then, correctly describes testosterone's bind to receptors embedded in the plasma membrane They are able to effect changes on the inside of the cell with the help of "second messengers," such as cAMP cAMP exerts its effects on the cell mainly by activating an enzyme that then causes a cascade of events resulting in cellular changes Choice D is wrong because testosterone does not inhibit Müllerian duct development and does promote the development of male structures Again, choice C is the correct answer Choice B is the correct answer This is your basic Mendelian genetics question, phrased in a way that might have tricked some of you into thinking this question was harder than it actually is During any individual pregnancy, the probability that the child will be male or female is 50-50, regardless of the sex of the previous child Even if a woman had given birth to 10 girls, her 11th child would still have only a 50% chance of being a girl Genotypically normal males produce sperm with either the X or Y chromosome, in a 50-50 proportion, while genotypically normal females produce ova with the X chromosome 100% of the time There is a 50% chance that a sperm carrying the X chromosome will fertilize the X-carrying ova, and a 50% chance that the fertilizing will be done by a sperm carrying a Y Thus, there is a 50% chance the fetus will be female, and a 50% chance it'll be male So, choice B is the correct answer Passage II (Questions 6-11) The correct answer is choice D Since the passage states that endometrial tissue outside the uterus is under the control of the same ovarian hormones that cause changes in the uterine endometrium, you can assume that the abnormal endometrial tissue will menstruate for the same reason as normal endometrium does Menstruation occurs when there is a sudden reduction in the levels of secretion of the ovarian hormones estrogen and progesterone This occurs approximately two weeks after ovulation Progesterone, in particular, is essential for the maintenance of the endometrium; the drop in progesterone causes the endometrium to slough off in menstruation A decrease in the secretion of progesterone and estrogen, therefore, is the hormonal change that induces the sloughing of the endometrial tissue outside the uterus Therefore, choice C is wrong and choice D is correct Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are gonadotropic hormones secreted by the anterior pituitary gland FSH causes maturation of ovarian follicles in women, while LH stimulates ovulation and formation of the corpus luteum, which develops from the ruptured ovarian follicle So, choices A and B are wrong Again, choice D is the right answer The correct answer is D Women on the pill are taking estrogen and progesterone, but not continuously At the appropriate time in their monthly cycle, the women either stop taking the pills, or take placebo pills instead This allows menstruation to occur and a new cycle to begin If they didn't, the high level of progesterone would prevent the endometrium from ever breaking down and sloughing off This is precisely why these hormones are used continuously to treat a patient with endometriosis: when estrogen and progesterone, in particular, are maintained at high levels, the abnormal endometrial tissue thickens but does not slough off and the bleeding is prevented Remember, it is the secretion of progesterone and estrogen following ovulation that causes the thickening of the endometrium in preparation for embryo implantation, and it is the decline in their secretion that causes menstruation Thus, choice D is the right answer Since progesterone also inhibits the secretion of gonadotropin-releasing hormone (GnRH), choice A has to be ruled out Inhibiting GnRH in turn inhibits the secretion of LH and FSH, so the high level of progesterone in a woman being treated for endometriosis does not allow the reproductive cycle to begin again as it would in a normal female The surge of LH that is essential in causing ovulation cannot occur either; therefore, there can be no "implantation of the ovum" (choice B) or "atrophy of the corpus luteum" (choice C) Again, choice D is the correct answer The correct answer is choice B Consider what each of the theories postulates is the source of the abnormal endometrial tissue The regurgitation theory holds that the tissue comes from menstrual backflow through the fallopian tubes Although this suffices as an explanation of the presence of endometrial tissue outside the uterus in the pelvic cavity, menstrual backflow could not possibly be the source of endometrial glands in the lungs or nasal mucosa The same line of reasoning can be leveled at the metaplastic theory: if the source of abnormal tissue were the lining of the abdominal cavity, one certainly wouldn't expect to find endometrial tissue as far away as the nasal mucosa Having ruled out options I and II, we have one option, III, and one choice, B, remaining The vascular or lymphatic dissemination theory, in contrast to the others, does provide a way to explain how endometrial tissue could be found so far away from the pelvic cavity; endometrial glands could be carried to the lungs, nose, or lymph nodes by the circulatory or lymphatic system This makes choice B the right answer The correct answer is choice A When fibrous tissue covers the ovaries and blocks the fallopian tubes, an ovum expelled from a follicle during ovulation will not be able to enter one of the fallopian tubes The ovum cannot come into 10 as developed by Reproductive System and Development Test contact with sperm, which means that fertilization cannot occur This is why women suffering from endometriosis are often infertile So, choice B is wrong, and choice A is correct Ovarian hormone function, choice C, is not suppressed in a woman with endometriosis until she starts to undergo drug treatment, so this could not be the cause of her infertility Inability to ovulate, choice D, is the most common cause of female sterility, but a woman with endometriosis can have normal ovulation and still be infertile Thus, choices C and D are wrong Again, choice A is the right answer 10 The correct answer is choice B You are asked to draw a conclusion based on two assumptions: 1) even normal women have regular menstrual backflow through the fallopian tubes; and 2) the regurgitation theory is correct in that endometriosis is the result of menstrual backflow If menstrual backflow through the fallopian tubes causes endometriosis, then why don't all women with menstrual backflow develop it? There must be something more involved; that is, there must be some factor or factors that make one woman prone to endometriosis and another woman resistant to it The factor or factors contributing to susceptibility are likely to be genetic or hormonal or immunological, so choice B is correct None of the other choices provides an answer to the puzzle of why only certain women would develop endometriosis from menstrual backflow Choice A has to with the metaplastic theory, not the regurgitation theory, so it's wrong There is nothing in the question to indicate that there are a "variety of conditions that cause irregular menstruation" based on the two assumptions mentioned earlier, so choice C is wrong too Endometriosis may be relatively common, but it obviously does not develop in "most women," so choice D is out Again, choice B is the right answer 11 The correct answer is choice A The removal of the ovaries would leave the woman devoid of estrogen and progesterone, a condition identical to that experienced by a normal woman after the menopause The menstrual cycle would cease altogether The lack of ovarian hormones would, contrary to the assertion of choice B, have a marked effect on all endometrial tissue It would no longer go through the cycle of thickening, secretory development, and sloughing off if the ovum is not fertilized Thus, choice B is wrong In the absence of estrogen and progesterone, there would be no inhibition of GnRH, LH, and FSH, so they would be produced in continuous quantities (this occurs at the menopause in the normal woman) This makes choice A the correct answer On the other hand, there is no reason to think that surgical removal must be limited to any time in the woman's monthly cycle, so choice C is clearly not the best answer Choice D is wrong because only the ovarian hormones would no longer be produced upon removal of the ovaries Production of the gonadotropic hormones LH and FSH is considerable increased due to the absence of negative feedback Again, choice A is the correct answer Discretes (Questions 12-16) 12 The correct answer is choice B Let's review the fate of each of the three embryonic germ layers Endoderm, choice A, develops into the epithelial lining of the digestive and respiratory tracts and parts of the liver, pancreas, thyroid and bladder Mesoderm, choice B, develops into the musculoskeletal system, the circulatory system, the excretory system, gonads, and the outer coverings of internal organs Epiderm, also known as epidermis, choice C, is not an embryonic germ layer, but rather the layer of the skin that overlies the dermis Ectoderm, choice D, develops into the brain and nervous system, the lens of the eye, the inner ear, hair and nails, sweat glands, the lining of the nose and mouth, and the epidermis of the skin Since mesoderm is responsible for the differential development of smooth muscle, choice B is the correct answer 13 The correct answer is choice C As the question stem illustrates, ectodermal cells have the potential at least in the embryonic state to develop into more than one type of tissue In the experiment, development of the ectodermal cells was influenced by changing their location, which means, contrary to choice A, that the fate of the cells had not yet been determined If the transplanted cells had developed into wing feathers instead of claws, it would indicate that the fate of the cells had been determined So, choice A is wrong The location of those ectodermal cells is so important because ectodermal cell development is induced by the underlying mesodermal cells, which send out chemical substances known as inducers that are specific to the position of the mesoderm This means that it's the mesoderm, not the ectoderm itself, that determines which types of cells are produced at this stage of development So, choice B is wrong and choice C is right While this may seem a bit counterintuitive, you might not have known that the underlying mesoderm has already differentiated, and that is why it is able to send out very specific signals to the overlying ectoderm, which is still undifferentiated Choice D can be ruled out because the assumption that ectodermal cells can develop into any type of tissue is not only wrong (see the explanation to the previous question), but it does not explain why location influences the development of the transplanted ectodermal cells Again, choice C is the correct answer 14 The correct answer is choice D The placenta performs quite a few important functions, and you should be familiar with them for the MCAT Choice A can be eliminated because the placenta does secrete human chorionic gonadotropin (hCG) the hormone that preserves the corpus luteum and thus the corpus luteum's production of estrogen and progesterone Note that the trophoblast cells of the developing embryo secrete hCG in quantities detectable in the blood to days after ovulation, around the time of implantation In fact, the presence of hCG is often used as a pregnancy detector Choice B is wrong because certain nutrients are actively absorbed from the maternal blood in the placenta, particularly during the first half KAPLAN 11 MCAT of pregnancy As for choice C, the only means that the fetus has of excreting carbon dioxide is through the placental membrane into the mother's blood However, the placenta is permeable to many drugs and toxins This creates the risk of abnormal development of the fetus consider the growth retardation of an infant with fetal alcohol syndrome or the low birth weights associated with infants whose mothers smoked during pregnancy Thus, choice D is the right answer 15 The correct answer is choice A Both mitosis and meiosis I are preceded by DNA replication, but meiosis II, the second stage of gametogenesis, is not Choice B is wrong because there are 23 pairs of chromosomes prior to meiosis I, not meiosis II The statement in choice C, that the polar body contains the haploid number of chromosomes, may be true, but it does not prove that the division was meiosis II and not meiosis I When a primary oocyte completes meiosis I, it yields a secondary oocyte and a polar body that also contains the haploid number of chromosomes So, choice C is wrong Choice D is wrong because after meiosis II, each of the chromosomes in the mature ovum consists of only one of the original sister chromatid pair Again, choice A is the right answer 16 The correct answer is choice C Choices A and B are not that tricky: the fact that fetal blood delivers almost the same amount of oxygen to fetal tissues as does maternal blood to the mother's tissues, despite a lower PO2 in the placenta, can clearly be explained by the fact that there is more fetal hemoglobin in fetal blood than there is adult hemoglobin in maternal blood and that fetal hemoglobin can carry more oxygen than maternal blood can for a given PO2 The latter occurs because fetal hemoglobin has a greater affinity for oxygen than does adult hemoglobin Remember that the hemoglobin-oxygen dissociation curve of fetal hemoglobin is to the left of that of adult hemoglobin Eliminating choice D requires a bit more thought The Bohr effect is the term used to describe hemoglobin's decreased capacity to carry oxygen as PCO2 increases So what does this mean in terms of the placenta? Well, fetal blood entering the placenta has a high concentration of CO2, most of which diffuses into the maternal blood in order to be delivered to the lungs for expulsion This means that, in the placenta, there is a higher PCO2 in the maternal blood than in the fetal blood Thus, the capacity of fetal hemoglobin to hold onto oxygen will be greater than the capacity of adult hemoglobin to hold onto oxygen So, while the capacity of both fetal and adult hemoglobin to transport oxygen is affected by PCO2, the effect is such that more oxygen is forced from the maternal blood, while enhancing the transport of oxygen in the fetal blood Thus, choice D is wrong So this leaves us with choice C Now while it is true that adult hemoglobin is produced after birth, this does not even come close to explaining how fetal blood transports as much oxygen as it does before birth So, choice C is our right answer 12 as developed by ... drugs and toxins END OF TEST KAPLAN MCAT ANSWER KEY: B A C C B 10 D D B A B 11 12 13 14 15 A B C D A 16 C as developed by Reproductive System and Development Test REPRODUCTIVE SYSTEM AND DEVELOPMENT. .. digestive and respiratory tracts and parts of the liver, pancreas, thyroid and bladder Mesoderm, choice B, develops into the musculoskeletal system, the circulatory system, the excretory system, ... Ectoderm, choice D, develops into the brain and nervous system, the lens of the eye, the inner ear, hair and nails, sweat glands, the lining of the nose and mouth, and the epidermis of the skin Since
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