Passage I Questions 1–5The process of sex differentiation involves the development of male or female characteristics, including differentiation of the gonads, the internal reproductive t
Trang 1BIOLOGY TOPICAL:
Reproductive System and Development
Test 1
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
Trang 2DIRECTIONS: 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
1
H
1.0
2
He
4.0 3
Li
6.9
4
Be
9.0
5
B
10.8
6
C
12.0
7
N
14.0
8
O
16.0
9
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.
Trang 3Passage 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
1 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
2 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
self-fertilization
3 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.
Trang 44 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
5 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 25%
B 50%
C 75%
D 100%
GO ON TO THE NEXT PAGE.
Trang 5Passage 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
6 The periodic bleeding of the non-uterine endometrial
tissue is directly induced by which of the following
hormonal changes?
A Increased secretion of FSH and LH
B Decreased secretion of FSH and LH
C Increased secretion of estrogen and progesterone
D Decreased secretion of estrogen and progesterone
7 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 secretion of gonadotropin-releasing hormone.
B implantation of the ovum.
C atrophy of the corpus luteum.
D thickening of the endometrium.
8 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 I only
B III only
C I and III only
D II and III only
9 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:
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.
Trang 61 0 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 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.
Trang 7Questions 12 through 16 are
NOT based on a descriptive
passage
1 2 Smooth muscle develops from which of the
following germ layers?
A Endoderm
B Mesoderm
C Epiderm
D Ectoderm
1 3 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 the fate of the cells had already been determined.
B ectodermal cells possess positional information.
C the cells were induced by underlying mesoderm.
D ectodermal cells can develop into any type of
tissue
1 4 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.
1 5 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
1 6 In the placenta, fetal blood has a PO 2 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
END OF TEST
Trang 8ANSWER KEY:
Trang 9REPRODUCTIVE SYSTEM AND DEVELOPMENT TEST 1 TRANSCRIPT
Passage 1 (Questions 1-5)
1 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 1 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
2 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, do 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 do 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
3 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 do 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
Trang 10molecules and pass through cell membranes by diffusion Steroids bind their receptor proteins inside the cell to form steroid-receptor complexes The binding of the hormone changes the steroid-receptor's conformation and increases the steroid-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
5 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)
6 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
7 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
8 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