Biology Reproductive

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MCAT Topical Tests
Dear Future Doctor,
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BIOLOGY 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.

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
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.

2

as developed by

Reproductive System and Development Test 1
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.

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 selffertilization.

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.

KAPLAN

3

MCAT
4 . 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.
B.
C.
D.

25%
50%
75%
100%

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4

as developed by

Reproductive System and Development Test 1
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.

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.
B.
C.
D.

secretion of gonadotropin-releasing hormone.
implantation of the ovum.
atrophy of the corpus luteum.
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.
B.
C.
D.

I only
III only
I and III only
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:
6 . 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.

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KAPLAN

5

MCAT
1 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.

6

as developed by

Reproductive System and Development Test 1
Questions 12 through 16 are
NOT based on a descriptive
passage.
1 2 . Smooth muscle develops from which of the
following germ layers?
A.
B.
C.
D.

Endoderm
Mesoderm
Epiderm
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.
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.

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 P O 2 of only 30
mmHg while maternal blood has a PO of 50 mmHg,
2
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 PO .
2
C . The newborn begins to produce adult hemoglobin
after birth.
D . The capacity of both fetal and adult hemoglobin
to carry oxygen decreases as PCO increases.
2

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.

END OF TEST

KAPLAN

7

MCAT
ANSWER KEY:
1.
B
6.
2.
A
7.
3.
C
8.
4.
C
9.
5.
B
10.

8

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 1
REPRODUCTIVE 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.
4.
The correct answer is choice C. Question 4 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

9

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 B is referring to peptide hormones, which 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.
9.
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 1
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 do 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 8 to 9 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

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