10-05-2006, 09:43 PM
A mutation affecting the development of the diencephalon could interfere with the secretion of which of
the following hormones?
A. Adrenocorticotrophic hormone (ACTH)
B. Epinephrine
C. Oxytocin
D. Prolactin
E. Thyroid stimulating hormone (TSH)
Explanation:
The correct answer is C. The neurohypophysis (posterior pituitary) is derived from an evagination of
diencephalic neurectoderm. This structure is responsible for releasing oxytocin and vasopressin
to the general circulation. Both hormones are synthesized in cell bodies contained within the
hypothalamus.
ACTH (choice A), prolactin (choice D), and TSH (choice E) are all synthesized and released by
the anterior pituitary, or adenohypophysis, which is derived from an evagination of the
ectoderm of Rathke's pouch, a diverticulum of the primitive mouth. Remnants of this pouch may
give rise to a craniopharyngioma in later life.
Epinephrine (choice B) is synthesized and released into the circulation by the adrenal medulla,
a neural crest derivative.
Which of the following characteristics is similar for spermatogenesis and oogenesis?
A. Age at which meiosis begins
B. Amount of cytoplasm retained
C. DNA replication during meiosis
D. Length of prophase I
E. Transmission to fetus of mitochondrial DNA
Explanation:
The correct answer is C. The actual process of manipulation of DNA and chromosomes during
meiosis is very similar in spermatogenesis and oogenesis. However, the processes also differ in many
other respects:
In oogenesis, the process of meiosis begins before birth, and arrests between birth and puberty
in prophase I. In contrast, spermatogenesis does not begin until puberty (see choice A).
The egg retains a large volume of cytoplasm (choice B), while nearly all the cytoplasm is
stripped during formation of a sperm.
As noted above, in oogenesis, meiosis is arrested in prophase I, which is consequently very
prolonged in the female. In spermatogenesis, meiosis is completed in a much shorter time (choice D).
Both the egg and the sperm have mitochondria, but those of the sperm are left outside when the
sperm nucleus enters the egg, and consequently do not contribute to the mitochondrial genome of
the fetus. Instead, the mitochondria are transmitted from the egg to the fetus (choice E).
Traits coded for by mitochondrial DNA are therefore inherited in a matrilineal fashion.
Which of the following respiratory system components is derived from neural crest?
A. Endothelial cells
B. Epithelium of primary bronchi
C. Laryngeal cartilage
D. Tracheal glands
E. Type I pneumocytes
Explanation:
The correct answer is C. Laryngeal cartilages (e.g., the thyroid, cricoid, arytenoid
cartilages) are derived from neural crest.
The endothelial cells (choice A) in the simple squamous epithelium that lines the pulmonary
capillaries are derived from visceral mesoderm.
The epithelial lining of primary bronchi (choice B) is derived from endoderm.
Tracheal glands (choice D) and epithelium both derive from endoderm.
Type I pneumocytes (choice E) are derived from endoderm.
The smooth part of the right atrium derives from which of the following embryonic structures?
A. Bulbus cordis
B. Primitive atrium
C. Primitive ventricle
D. Sinus venosus
E. Truncus arteriosus
Explanation:
The correct answer is D. The smooth part of the right atrium (the sinus venarum) is derived
from the sinus venosus. The coronary sinus and the oblique vein of the left atrium also derive
from the sinus venosus.
The bulbus cordis (choice A) gives rise to the smooth part of the right ventricle (conus
arteriosus) and the smooth part of the left ventricle (aortic vestibule).
The primitive atrium (choice B) gives rise to the trabeculated part of the right and left atria.
The primitive ventricle (choice C) gives rise to the trabeculated part of the right and left
ventricles.
The truncus arteriosus (choice E) gives rise to the proximal part of the aorta and the
proximal part of the pulmonary artery.
A patient, who appears to be female, is found to be 46,XY. The patient's vagina is very shallow, ending
in a blind pouch, and there are palpable masses in the labia. The diagnosis of testicular
feminization syndrome is made. Which of the following was most likely present during the early
fetal life of this individual?
A. A streak ovary
B. A uterus
C. An oviduct
D. Depressed levels of testosterone
E. MIF (Mullerian inhibitory factor)
Explanation:
The correct answer is E. Testicular feminization is a disorder of the androgen receptor.
Phenotypically, the patient appears female, but has a blindly ending vagina and lacks a uterus or other
female internal reproductive organs. The patient has an XY genotype. Since the gene for testes
determining factor (TDF) is on the Y chromosome, TDF will cause the indifferent gonad to develop into a
testis containing Sertoli cells. Sertoli cells at this stage will secrete MIF, a substance that
suppresses the paramesonephric ducts, preventing the formation of female internal reproductive organs.
This patient would not have a streak ovary (choice A), a finding in Turner's syndrome that is
associated with a 45,XO genotype. In fact, the patient would have testes, since the genetic complement
contained a Y chromosome. The testes in individuals with testicular feminization syndrome are often
undescended and are usually removed surgically.
This patient would not possess a uterus (choice B) or an oviduct (choice C). Both of these
structures are derived from the paramesonephric duct, which is suppressed by MIF.
Depressed levels of testosterone (choice D) would not occur in this patient. In fact,
individuals with testicular feminization syndrome have normal or even slightly elevated levels
of testosterone. The development of female external genitalia is the result of defective
androgen receptors, not depressed levels of testosterone.
At which of the following ages does fetal movement first occur?
A. 1 month
B. 2 months
C. 4 months
D. 6 months
E. 7 months
Explanation:
The correct answer is B. Neuromuscular development is sufficient to allow fetal movement in the
eighth week of life. Other features of week 8 include the first appearance of a thin skin, a
head as large as the rest of the body, forward-looking eyes, appearance of digits on the hands
and feet, appearance of testes and ovaries (but not distinguishable external genitalia), and a
crown-rump length of approximately 30 mm. By the end of the eighth week, nearly all adult
structures have at least begun to develop, and the fetus "looks like a baby".
A neonate is found to have a sacrococcygeal teratoma that contains several different tissue types
resulting from a persistence of the primitive streak. The primitive streak normally gives rise
to which of the following structures?
A. Dorsal root ganglia
B. Lining of the gastrointestinal tract
C. Notochord
D. Spinal cord
E. Thyroid gland
Explanation:
The correct answer is C. The primitive streak is the region of the epiblast through which the
cells that give rise to the notochord and the mesoderm of the embryo pass. The only adult
derivative of the notochord is the nucleus pulposus of the intervertebral disk. The mesoderm
gives rise to many different tissue types including muscle, connective tissue, and blood. This
accounts for the many different tissue types found in a sacrococcygeal teratoma.
The dorsal root ganglia (choice A) are derived from neural crest cells. The neural crest
develops at the time of neurulation, which is a process of infolding of the neural ectoderm that is
induced by the notochord.
The lining of the gastrointestinal tract (choice B) is derived from endoderm. The endoderm also
gives rise to evaginations of the gastrointestinal tract, such as the liver and pancreas. The
smooth muscle and connective tissue of the gastrointestinal tract are derived from mesoderm.
The spinal cord (choice D) is derived from neural ectoderm, which invaginates during
neurulation to form the neural tube. The caudal part of the neural tube forms the spinal cord,
and the rostral part of the neural tube forms the brain.
The thyroid gland (choice E) is derived from endoderm. The endoderm of the floor of the pharynx
evaginates to form the thyroglossal duct, which descends to form the thyroid gland. The adult
site of the evagination of the thyroglossal duct is marked by the foramen cecum on the tongue.
An infant is born with hydrocephalus. CT studies demonstrate herniation of the cerebellum into the
spinal canal. This is an example of which of the following?
A. Arnold-Chiari malformation
B. Dandy-Walker malformation
C. Holoprosencephaly
D. Lissencephaly
E. Porencephaly
Explanation:
The correct answer is A. Arnold-Chiari malformations are congenital herniations of hindbrain
structures into the spinal canal. In type I Arnold-Chiari malformation, the cerebellar tonsils
herniate into the foramen magnum; in type II, parts of the hindbrain, cerebellar vermis, and
fourth ventricle herniate into the foramen. This second type is commonly associated with
meningomyeloceles. Other neonatal presentations associated with Arnold-Chiari malformation
include hydrocephalus (because of cerebrospinal fluid obstruction) and brainstem dysfunction
(causing stridor and poor swallowing). Milder cases may present later in life with spinal cord
or cerebellar symptoms.
Dandy-Walker malformation (choice B) is associated with hypoplasia of the cerebellum associated
with an enlarged fourth ventricle.
Holoprosencephaly (choice C) is a complex malformation of the brain such that only a single large
monoventricular cerebral hemisphere exists.
Lissencephaly (choice D), or agyria, means "smooth brain" and is a condition characterized by a
lack of hemispheric sulci and severe neurologic impairment.
Porencephaly (choice E) is a severe cleft in the brain that allows the ventricles to
communicate with the subarachnoid space.
During anatomy lab, a medical student notes a fibrous band that runs on the visceral surface of the
liver. It is attached on one end to the inferior vena cava and on the other end to the left
branch of the portal vein. In the embryo, this structure corresponds to the
A. Ductus venosus
B. Ligamentum teres
C. Ligamentum venosum
D. Umbilical arteries
E. Umbilical vein
Explanation:
The correct answer is A. This question could have tricked you if you didn't catch the key
words, "in the embryo." If you read the question too quickly and thought you were going to be
asked to identify the structure described, you probably chose choice C (ligamentum venosum),
since that is indeed the structure in question. However, in the embryo, this fibrous band is
actually the ductus venosus. The ductus venosus is an embryonic vessel that allows blood to
bypass the fetal liver; this prevents the depletion of oxygen and nutrient-rich blood in the
hepatic sinusoids.
The embryonic umbilical vein (choice E) actually becomes the fibrous ligamentum teres (choice B). The
ligamentum teres is located in the free margin of the falciform ligament.
The embryonic umbilical arteries (choice D) become the medial umbilical ligaments.
A newborn boy does not pass meconium until 48 hours after his birth. Several weeks later his mother
complains that he has not been passing stool regularly. Anorectal manometry reveals increased
internal anal sphincter pressure on rectal distention with a balloon. The patient's disorder
may be attributed to
A. defective recanalization of the colon
B. failure of neural crest cells to migrate into the colonic wall
C. herniation of abdominal contents into the umbilical cord
D. persistence of the proximal end of the yolk stalk
E. presence of a rectourinary fistula
Explanation:
The correct answer is B. The baby boy has Hirschsprung's disease (more common in males), a
diagnosis that should spring to mind in an infant who fails to pass meconium soon after birth
and presents with chronic constipation. Diagnosis is made most easily and most reliably by anal
manometry and/or rectal biopsy. On manometry, internal anal sphincter pressure increases upon
rectal balloon distention in patients with Hirschsprung's disease (normally, pressure decreases
with distention). Rectal biopsy reveals an absence of ganglion cells (which are derived from
neural crest cells) in a portion of the colonic wall. Barium enema would show a transition zone
between the aganglionic area of bowel (narrow) and the region of normal bowel proximal to it
(dilated).
Defective recanalization of the colon (choice A) results in rectal atresia. In this condition,
there is no communication between the rectum and anal canal. Therefore, neither meconium nor stool can
be passed.
Herniation of abdominal contents into the umbilical cord (choice C) describes a congenital
omphalocele. This occurs due to the failure of all or part of the intestines to return into the
abdominal cavity during the tenth week of gestation.
Persistence of the proximal end of the yolk stalk (choice D) results in Meckel's diverticulum,
an ileal outpouching that is more prevalent in males. It usually contains ectopic gastric
mucosa, which can cause ulceration and bleeding. Inflammation of the diverticulum may produce
symptoms and signs similar to appendicitis.
The presence of a rectourinary fistula (choice E) would result in the passage of meconium,
stool, and gas into the urine, a diagnostic finding.
Which of the pharyngeal pouches develops into the palatine tonsil?
A. First
B. Second
C. Third
D. Fourth
E. Fifth
Explanation:
The correct answer is B. The epithelial lining of the second pharyngeal pouch buds into the
mesenchyme to form the palatine tonsil. Part of the pouch remains in the adult as the tonsillar fossa.
It is important to review the other choices since pharyngeal pouch derivatives are typically
tested on the USMLE Step 1:
The first pharyngeal pouch (choice A) develops into the middle ear cavity and eustachian tube.
The third pharyngeal pouch (choice C) develops into the thymus and the inferior parathyroid glands.
The fourth pharyngeal pouch (choice D) gives rise to the superior parathyroid glands. Recall
that abnormal development of the 3rd and 4th pouches leads to DiGeorge syndrome and results in
hypocalcemia as well as abnormal cellular immunity and consequent susceptibility to viral and
fungal illnesses.
The fifth pharyngeal pouch (choice E) gives rise to the C cells of the thyroid gland. These
cells secrete calcitonin–a hormone that lowers serum calcium.
A 5-year-old child is brought to the emergency room with massive, painless bleeding from the rectum.
Colonoscopy fails to demonstrate a lesion in the colon or anus. Upper endoscopy fails to demonstrate
esophagitis, gastric ulcer, or duodenal ulcer. A [99mTc] technetium scan demonstrates an abnormality in
the lower half of the abdomen. Failure of a normal developmental process involving which of the
following structures is the most likely cause of this child's bleeding?
A. Appendix
B. Cecum
C. Duodenum
D. Ileum
E. Jejunum
Explanation:
The correct answer is D. A Meckel's diverticulum is caused by failure of obliteration of the
vitello-intestinal duct. It is classically located in the distal ileum within 30 cm of the
ileocecal valve, and the structure is a true diverticulum with mucosa, submucosa, and
muscularis propria. Many Meckel's diverticula contain ectopic pancreatic tissue or gastric
mucosa, and the acid production from the gastric mucosa may be sufficient to produce a small
peptic ulcer in adjacent intestinal mucosa. Such small peptic ulcers are occasional sources of
mysterious appendicitis-like pain or intestinal bleeding. Peptic ulceration adjacent to a
Meckel's diverticulum should be suspected in any child who presents with massive, painless
rectal bleeding. Technetium [99mTc] concentrates in gastric mucosa, and the scan in this
patient demonstrated a small amount of ectopic gastric mucosa located in the diverticulum.
Acute appendicitis (choice A) is usually very painful and does not typically cause rectal bleeding.
A lesion of the cecum (choice B) would have been revealed by thorough colonoscopy.
Failure of upper endoscopy to demonstrate a peptic ulcer of the duodenum (choice C) makes duodenal
disease unlikely.
Theoretically, the jejunum (choice E) could have been the source of the problems, but jejunal
bleeding is uncommon and a Meckel's diverticulum is a much more likely possibility.
A 12-month-old child is diagnosed with an atrial septal defect. What is the most common cause of such a
congenital heart malformation?
A. Failure of formation of the septum primum
B. Failure of formation of the septum secundum
C. Incomplete adhesion between the septum primum and septum secundum
D. Malformation of the membranous interventricular septum
E. Malformation of the muscular interventricular septum
Explanation:
The correct answer is C. The most common form of atrial septal defect is located near the
foramen ovale (not to be confused with a patent foramen ovale, which is of little or no
hemodynamic significance). They result from incomplete adhesion between the septum primum and
the septum secundum during development.
Atrial septal defects less commonly result from failures of formation of the septum primum
(choice A) and septum secundum (choice B).
Malformations of the interventricular septum (choices D and E) cause ventricular septal defects
rather than atrial septal defects.
A newborn male child is noted to have hypospadias. A complete evaluation determines that the child has
no other genitourinary anomalies. Nonetheless, hypospadias repair will be performed to prevent
which of the following possible sequelae?
A. Bladder exstrophy
B. Hydrocele
C. Phimosis
D. Urachal cysts
E. Urinary tract infection
Explanation:
The correct answer is E. Hypospadias, which is congenital displacement of the urethral opening
onto the ventral surface (underside) of the penis, is due to malformation of the urethral
groove and canal. Hypospadias frequently accompanies other genitourinary anomalies, especially
cryptorchidism. Isolated hypospadias is repaired because the abnormal opening is often
constricted, leading to urinary retention and ascending urinary tract infections. Another
important consequence of hypospadias is sterility, which occurs if the opening is too close to
the base of the penis to permit normal ejaculation. Although hypospadias can occur in
isolation, it is strongly associated with other urogenital anomalies.
Bladder exstrophy (choice A) is a completely unrelated congenital malformation in which the
abdominal wall and anterior bladder wall form incompletely, and the bladder mucosa is exposed to the
environment.
Hydrocele (choice B) is a serous accumulation in the tunica vaginalis, often producing a
readily apparent scrotal mass. Hydrocele usually arises without any obvious cause.
Phimosis (choice C) is the condition in which the foreskin cannot be retracted over the head of
the penis. It is usually either a congenital malformation or a consequence of scarring.
Urachal cysts (choice D) are produced by areas of persistent urachal epithelium, which may be present
anywhere between the bladder and the umbilicus.
A 25-year-old female presents to her obstetrician after taking a home pregnancy test with a positive
result. She states that twins run in her family and would like an ultrasound to determine if
she has a twin pregnancy. Radiographic studies confirm that the embryo has split at the
blastocyst stage. Splitting of the embryo at the blastocyst stage results in which of the
following?
A. Conjoined twins
B. Dizygotic twins
C. Fraternal twins
D. Monozygotic twins
E. Single gestation
Explanation:
The correct answer is D. Monozygotic twins, or identical twins, develop from a single
fertilized egg that subsequently splits during either the blastomere or blastocyst stage. It is
more common in the blastocyst stage at the end of the first week. This results in two inner
cell masses in the same blastocyst cavity. They usually develop a common placenta and chorionic
cavities but separate amniotic cavities. Splitting in the second week usually results in shared
amniotic cavities as well.
Conjoined twins (choice A), or Siamese twins, result from incomplete splitting of the embryo.
Dizygotic twins (choice B) and fraternal twins (choice C) are the same, and are the most common
type of twins. They share the same genetic relatedness as do siblings of separate pregnancies.
This type of twinning occurs because of simultaneous double ovulation followed by fertilization
by two sperm. They each develop their own placenta and membranes.
A single gestation (choice E), and a single birth, is the result of a single fertilization
without any splitting of the embryo.
On physical examination of a 7-year-old boy, the child's upper body appears much more developed than his
lower body. Blood pressure in the upper extremities exceeds that of the lower extremities. On cardiac
examination, there is a midsystolic murmur over the anterior chest and back. The child's lower
extremities are cold, and femoral pulses are absent. The part of the vascular system that is affected in
this disorder is derived from which of the following embryologic structures?
A. Bulbus cordis
B. Ductus arteriosus
C. Left horn of sinus venosus
D. Right common cardinal vein
E. Right horn of sinus venosus
F. Third, fourth, and sixth aortic arches
Explanation:
The correct answer is F. This patient has coarctation of the aorta (constriction of the
ascending aorta), which is suggested by a midsystolic murmur over the anterior part of the
chest and back, hypertension in the upper extremities, and absent or delayed pulsations in the
femoral arteries. The upper extremities and thorax may be more developed than the lower
extremities. Patients with coarctation of the aorta may experience symptoms such as cold
extremities as a result of tissue ischemia.The truncus arteriosus gives rise to the proximal
portions of the ascending aorta and the pulmonary trunk. The third, 4th, and 6th aortic arches
and the right and left dorsal aortae contribute to the remainder of the aorta.
The bulbus cordis (choice A) gives rise to the right ventricle and the aortic outflow tract.
98% of cases of coarctation of the aorta take place immediately distal to the offshoot of the
left subclavian artery, close to the junction of the ductus arteriosus (choice B) with the aorta.
The left horn of the sinus venosus (choice C) gives rise to the coronary sinus.
The right common cardinal vein (choice D) gives rise to the superior vena cava.
The right horn of the sinus venosus (choice E) gives rise to the smooth part of the right atrium.
ok, i think these are 50+ que's.....GL.
the following hormones?
A. Adrenocorticotrophic hormone (ACTH)
B. Epinephrine
C. Oxytocin
D. Prolactin
E. Thyroid stimulating hormone (TSH)
Explanation:
The correct answer is C. The neurohypophysis (posterior pituitary) is derived from an evagination of
diencephalic neurectoderm. This structure is responsible for releasing oxytocin and vasopressin
to the general circulation. Both hormones are synthesized in cell bodies contained within the
hypothalamus.
ACTH (choice A), prolactin (choice D), and TSH (choice E) are all synthesized and released by
the anterior pituitary, or adenohypophysis, which is derived from an evagination of the
ectoderm of Rathke's pouch, a diverticulum of the primitive mouth. Remnants of this pouch may
give rise to a craniopharyngioma in later life.
Epinephrine (choice B) is synthesized and released into the circulation by the adrenal medulla,
a neural crest derivative.
Which of the following characteristics is similar for spermatogenesis and oogenesis?
A. Age at which meiosis begins
B. Amount of cytoplasm retained
C. DNA replication during meiosis
D. Length of prophase I
E. Transmission to fetus of mitochondrial DNA
Explanation:
The correct answer is C. The actual process of manipulation of DNA and chromosomes during
meiosis is very similar in spermatogenesis and oogenesis. However, the processes also differ in many
other respects:
In oogenesis, the process of meiosis begins before birth, and arrests between birth and puberty
in prophase I. In contrast, spermatogenesis does not begin until puberty (see choice A).
The egg retains a large volume of cytoplasm (choice B), while nearly all the cytoplasm is
stripped during formation of a sperm.
As noted above, in oogenesis, meiosis is arrested in prophase I, which is consequently very
prolonged in the female. In spermatogenesis, meiosis is completed in a much shorter time (choice D).
Both the egg and the sperm have mitochondria, but those of the sperm are left outside when the
sperm nucleus enters the egg, and consequently do not contribute to the mitochondrial genome of
the fetus. Instead, the mitochondria are transmitted from the egg to the fetus (choice E).
Traits coded for by mitochondrial DNA are therefore inherited in a matrilineal fashion.
Which of the following respiratory system components is derived from neural crest?
A. Endothelial cells
B. Epithelium of primary bronchi
C. Laryngeal cartilage
D. Tracheal glands
E. Type I pneumocytes
Explanation:
The correct answer is C. Laryngeal cartilages (e.g., the thyroid, cricoid, arytenoid
cartilages) are derived from neural crest.
The endothelial cells (choice A) in the simple squamous epithelium that lines the pulmonary
capillaries are derived from visceral mesoderm.
The epithelial lining of primary bronchi (choice B) is derived from endoderm.
Tracheal glands (choice D) and epithelium both derive from endoderm.
Type I pneumocytes (choice E) are derived from endoderm.
The smooth part of the right atrium derives from which of the following embryonic structures?
A. Bulbus cordis
B. Primitive atrium
C. Primitive ventricle
D. Sinus venosus
E. Truncus arteriosus
Explanation:
The correct answer is D. The smooth part of the right atrium (the sinus venarum) is derived
from the sinus venosus. The coronary sinus and the oblique vein of the left atrium also derive
from the sinus venosus.
The bulbus cordis (choice A) gives rise to the smooth part of the right ventricle (conus
arteriosus) and the smooth part of the left ventricle (aortic vestibule).
The primitive atrium (choice B) gives rise to the trabeculated part of the right and left atria.
The primitive ventricle (choice C) gives rise to the trabeculated part of the right and left
ventricles.
The truncus arteriosus (choice E) gives rise to the proximal part of the aorta and the
proximal part of the pulmonary artery.
A patient, who appears to be female, is found to be 46,XY. The patient's vagina is very shallow, ending
in a blind pouch, and there are palpable masses in the labia. The diagnosis of testicular
feminization syndrome is made. Which of the following was most likely present during the early
fetal life of this individual?
A. A streak ovary
B. A uterus
C. An oviduct
D. Depressed levels of testosterone
E. MIF (Mullerian inhibitory factor)
Explanation:
The correct answer is E. Testicular feminization is a disorder of the androgen receptor.
Phenotypically, the patient appears female, but has a blindly ending vagina and lacks a uterus or other
female internal reproductive organs. The patient has an XY genotype. Since the gene for testes
determining factor (TDF) is on the Y chromosome, TDF will cause the indifferent gonad to develop into a
testis containing Sertoli cells. Sertoli cells at this stage will secrete MIF, a substance that
suppresses the paramesonephric ducts, preventing the formation of female internal reproductive organs.
This patient would not have a streak ovary (choice A), a finding in Turner's syndrome that is
associated with a 45,XO genotype. In fact, the patient would have testes, since the genetic complement
contained a Y chromosome. The testes in individuals with testicular feminization syndrome are often
undescended and are usually removed surgically.
This patient would not possess a uterus (choice B) or an oviduct (choice C). Both of these
structures are derived from the paramesonephric duct, which is suppressed by MIF.
Depressed levels of testosterone (choice D) would not occur in this patient. In fact,
individuals with testicular feminization syndrome have normal or even slightly elevated levels
of testosterone. The development of female external genitalia is the result of defective
androgen receptors, not depressed levels of testosterone.
At which of the following ages does fetal movement first occur?
A. 1 month
B. 2 months
C. 4 months
D. 6 months
E. 7 months
Explanation:
The correct answer is B. Neuromuscular development is sufficient to allow fetal movement in the
eighth week of life. Other features of week 8 include the first appearance of a thin skin, a
head as large as the rest of the body, forward-looking eyes, appearance of digits on the hands
and feet, appearance of testes and ovaries (but not distinguishable external genitalia), and a
crown-rump length of approximately 30 mm. By the end of the eighth week, nearly all adult
structures have at least begun to develop, and the fetus "looks like a baby".
A neonate is found to have a sacrococcygeal teratoma that contains several different tissue types
resulting from a persistence of the primitive streak. The primitive streak normally gives rise
to which of the following structures?
A. Dorsal root ganglia
B. Lining of the gastrointestinal tract
C. Notochord
D. Spinal cord
E. Thyroid gland
Explanation:
The correct answer is C. The primitive streak is the region of the epiblast through which the
cells that give rise to the notochord and the mesoderm of the embryo pass. The only adult
derivative of the notochord is the nucleus pulposus of the intervertebral disk. The mesoderm
gives rise to many different tissue types including muscle, connective tissue, and blood. This
accounts for the many different tissue types found in a sacrococcygeal teratoma.
The dorsal root ganglia (choice A) are derived from neural crest cells. The neural crest
develops at the time of neurulation, which is a process of infolding of the neural ectoderm that is
induced by the notochord.
The lining of the gastrointestinal tract (choice B) is derived from endoderm. The endoderm also
gives rise to evaginations of the gastrointestinal tract, such as the liver and pancreas. The
smooth muscle and connective tissue of the gastrointestinal tract are derived from mesoderm.
The spinal cord (choice D) is derived from neural ectoderm, which invaginates during
neurulation to form the neural tube. The caudal part of the neural tube forms the spinal cord,
and the rostral part of the neural tube forms the brain.
The thyroid gland (choice E) is derived from endoderm. The endoderm of the floor of the pharynx
evaginates to form the thyroglossal duct, which descends to form the thyroid gland. The adult
site of the evagination of the thyroglossal duct is marked by the foramen cecum on the tongue.
An infant is born with hydrocephalus. CT studies demonstrate herniation of the cerebellum into the
spinal canal. This is an example of which of the following?
A. Arnold-Chiari malformation
B. Dandy-Walker malformation
C. Holoprosencephaly
D. Lissencephaly
E. Porencephaly
Explanation:
The correct answer is A. Arnold-Chiari malformations are congenital herniations of hindbrain
structures into the spinal canal. In type I Arnold-Chiari malformation, the cerebellar tonsils
herniate into the foramen magnum; in type II, parts of the hindbrain, cerebellar vermis, and
fourth ventricle herniate into the foramen. This second type is commonly associated with
meningomyeloceles. Other neonatal presentations associated with Arnold-Chiari malformation
include hydrocephalus (because of cerebrospinal fluid obstruction) and brainstem dysfunction
(causing stridor and poor swallowing). Milder cases may present later in life with spinal cord
or cerebellar symptoms.
Dandy-Walker malformation (choice B) is associated with hypoplasia of the cerebellum associated
with an enlarged fourth ventricle.
Holoprosencephaly (choice C) is a complex malformation of the brain such that only a single large
monoventricular cerebral hemisphere exists.
Lissencephaly (choice D), or agyria, means "smooth brain" and is a condition characterized by a
lack of hemispheric sulci and severe neurologic impairment.
Porencephaly (choice E) is a severe cleft in the brain that allows the ventricles to
communicate with the subarachnoid space.
During anatomy lab, a medical student notes a fibrous band that runs on the visceral surface of the
liver. It is attached on one end to the inferior vena cava and on the other end to the left
branch of the portal vein. In the embryo, this structure corresponds to the
A. Ductus venosus
B. Ligamentum teres
C. Ligamentum venosum
D. Umbilical arteries
E. Umbilical vein
Explanation:
The correct answer is A. This question could have tricked you if you didn't catch the key
words, "in the embryo." If you read the question too quickly and thought you were going to be
asked to identify the structure described, you probably chose choice C (ligamentum venosum),
since that is indeed the structure in question. However, in the embryo, this fibrous band is
actually the ductus venosus. The ductus venosus is an embryonic vessel that allows blood to
bypass the fetal liver; this prevents the depletion of oxygen and nutrient-rich blood in the
hepatic sinusoids.
The embryonic umbilical vein (choice E) actually becomes the fibrous ligamentum teres (choice B). The
ligamentum teres is located in the free margin of the falciform ligament.
The embryonic umbilical arteries (choice D) become the medial umbilical ligaments.
A newborn boy does not pass meconium until 48 hours after his birth. Several weeks later his mother
complains that he has not been passing stool regularly. Anorectal manometry reveals increased
internal anal sphincter pressure on rectal distention with a balloon. The patient's disorder
may be attributed to
A. defective recanalization of the colon
B. failure of neural crest cells to migrate into the colonic wall
C. herniation of abdominal contents into the umbilical cord
D. persistence of the proximal end of the yolk stalk
E. presence of a rectourinary fistula
Explanation:
The correct answer is B. The baby boy has Hirschsprung's disease (more common in males), a
diagnosis that should spring to mind in an infant who fails to pass meconium soon after birth
and presents with chronic constipation. Diagnosis is made most easily and most reliably by anal
manometry and/or rectal biopsy. On manometry, internal anal sphincter pressure increases upon
rectal balloon distention in patients with Hirschsprung's disease (normally, pressure decreases
with distention). Rectal biopsy reveals an absence of ganglion cells (which are derived from
neural crest cells) in a portion of the colonic wall. Barium enema would show a transition zone
between the aganglionic area of bowel (narrow) and the region of normal bowel proximal to it
(dilated).
Defective recanalization of the colon (choice A) results in rectal atresia. In this condition,
there is no communication between the rectum and anal canal. Therefore, neither meconium nor stool can
be passed.
Herniation of abdominal contents into the umbilical cord (choice C) describes a congenital
omphalocele. This occurs due to the failure of all or part of the intestines to return into the
abdominal cavity during the tenth week of gestation.
Persistence of the proximal end of the yolk stalk (choice D) results in Meckel's diverticulum,
an ileal outpouching that is more prevalent in males. It usually contains ectopic gastric
mucosa, which can cause ulceration and bleeding. Inflammation of the diverticulum may produce
symptoms and signs similar to appendicitis.
The presence of a rectourinary fistula (choice E) would result in the passage of meconium,
stool, and gas into the urine, a diagnostic finding.
Which of the pharyngeal pouches develops into the palatine tonsil?
A. First
B. Second
C. Third
D. Fourth
E. Fifth
Explanation:
The correct answer is B. The epithelial lining of the second pharyngeal pouch buds into the
mesenchyme to form the palatine tonsil. Part of the pouch remains in the adult as the tonsillar fossa.
It is important to review the other choices since pharyngeal pouch derivatives are typically
tested on the USMLE Step 1:
The first pharyngeal pouch (choice A) develops into the middle ear cavity and eustachian tube.
The third pharyngeal pouch (choice C) develops into the thymus and the inferior parathyroid glands.
The fourth pharyngeal pouch (choice D) gives rise to the superior parathyroid glands. Recall
that abnormal development of the 3rd and 4th pouches leads to DiGeorge syndrome and results in
hypocalcemia as well as abnormal cellular immunity and consequent susceptibility to viral and
fungal illnesses.
The fifth pharyngeal pouch (choice E) gives rise to the C cells of the thyroid gland. These
cells secrete calcitonin–a hormone that lowers serum calcium.
A 5-year-old child is brought to the emergency room with massive, painless bleeding from the rectum.
Colonoscopy fails to demonstrate a lesion in the colon or anus. Upper endoscopy fails to demonstrate
esophagitis, gastric ulcer, or duodenal ulcer. A [99mTc] technetium scan demonstrates an abnormality in
the lower half of the abdomen. Failure of a normal developmental process involving which of the
following structures is the most likely cause of this child's bleeding?
A. Appendix
B. Cecum
C. Duodenum
D. Ileum
E. Jejunum
Explanation:
The correct answer is D. A Meckel's diverticulum is caused by failure of obliteration of the
vitello-intestinal duct. It is classically located in the distal ileum within 30 cm of the
ileocecal valve, and the structure is a true diverticulum with mucosa, submucosa, and
muscularis propria. Many Meckel's diverticula contain ectopic pancreatic tissue or gastric
mucosa, and the acid production from the gastric mucosa may be sufficient to produce a small
peptic ulcer in adjacent intestinal mucosa. Such small peptic ulcers are occasional sources of
mysterious appendicitis-like pain or intestinal bleeding. Peptic ulceration adjacent to a
Meckel's diverticulum should be suspected in any child who presents with massive, painless
rectal bleeding. Technetium [99mTc] concentrates in gastric mucosa, and the scan in this
patient demonstrated a small amount of ectopic gastric mucosa located in the diverticulum.
Acute appendicitis (choice A) is usually very painful and does not typically cause rectal bleeding.
A lesion of the cecum (choice B) would have been revealed by thorough colonoscopy.
Failure of upper endoscopy to demonstrate a peptic ulcer of the duodenum (choice C) makes duodenal
disease unlikely.
Theoretically, the jejunum (choice E) could have been the source of the problems, but jejunal
bleeding is uncommon and a Meckel's diverticulum is a much more likely possibility.
A 12-month-old child is diagnosed with an atrial septal defect. What is the most common cause of such a
congenital heart malformation?
A. Failure of formation of the septum primum
B. Failure of formation of the septum secundum
C. Incomplete adhesion between the septum primum and septum secundum
D. Malformation of the membranous interventricular septum
E. Malformation of the muscular interventricular septum
Explanation:
The correct answer is C. The most common form of atrial septal defect is located near the
foramen ovale (not to be confused with a patent foramen ovale, which is of little or no
hemodynamic significance). They result from incomplete adhesion between the septum primum and
the septum secundum during development.
Atrial septal defects less commonly result from failures of formation of the septum primum
(choice A) and septum secundum (choice B).
Malformations of the interventricular septum (choices D and E) cause ventricular septal defects
rather than atrial septal defects.
A newborn male child is noted to have hypospadias. A complete evaluation determines that the child has
no other genitourinary anomalies. Nonetheless, hypospadias repair will be performed to prevent
which of the following possible sequelae?
A. Bladder exstrophy
B. Hydrocele
C. Phimosis
D. Urachal cysts
E. Urinary tract infection
Explanation:
The correct answer is E. Hypospadias, which is congenital displacement of the urethral opening
onto the ventral surface (underside) of the penis, is due to malformation of the urethral
groove and canal. Hypospadias frequently accompanies other genitourinary anomalies, especially
cryptorchidism. Isolated hypospadias is repaired because the abnormal opening is often
constricted, leading to urinary retention and ascending urinary tract infections. Another
important consequence of hypospadias is sterility, which occurs if the opening is too close to
the base of the penis to permit normal ejaculation. Although hypospadias can occur in
isolation, it is strongly associated with other urogenital anomalies.
Bladder exstrophy (choice A) is a completely unrelated congenital malformation in which the
abdominal wall and anterior bladder wall form incompletely, and the bladder mucosa is exposed to the
environment.
Hydrocele (choice B) is a serous accumulation in the tunica vaginalis, often producing a
readily apparent scrotal mass. Hydrocele usually arises without any obvious cause.
Phimosis (choice C) is the condition in which the foreskin cannot be retracted over the head of
the penis. It is usually either a congenital malformation or a consequence of scarring.
Urachal cysts (choice D) are produced by areas of persistent urachal epithelium, which may be present
anywhere between the bladder and the umbilicus.
A 25-year-old female presents to her obstetrician after taking a home pregnancy test with a positive
result. She states that twins run in her family and would like an ultrasound to determine if
she has a twin pregnancy. Radiographic studies confirm that the embryo has split at the
blastocyst stage. Splitting of the embryo at the blastocyst stage results in which of the
following?
A. Conjoined twins
B. Dizygotic twins
C. Fraternal twins
D. Monozygotic twins
E. Single gestation
Explanation:
The correct answer is D. Monozygotic twins, or identical twins, develop from a single
fertilized egg that subsequently splits during either the blastomere or blastocyst stage. It is
more common in the blastocyst stage at the end of the first week. This results in two inner
cell masses in the same blastocyst cavity. They usually develop a common placenta and chorionic
cavities but separate amniotic cavities. Splitting in the second week usually results in shared
amniotic cavities as well.
Conjoined twins (choice A), or Siamese twins, result from incomplete splitting of the embryo.
Dizygotic twins (choice B) and fraternal twins (choice C) are the same, and are the most common
type of twins. They share the same genetic relatedness as do siblings of separate pregnancies.
This type of twinning occurs because of simultaneous double ovulation followed by fertilization
by two sperm. They each develop their own placenta and membranes.
A single gestation (choice E), and a single birth, is the result of a single fertilization
without any splitting of the embryo.
On physical examination of a 7-year-old boy, the child's upper body appears much more developed than his
lower body. Blood pressure in the upper extremities exceeds that of the lower extremities. On cardiac
examination, there is a midsystolic murmur over the anterior chest and back. The child's lower
extremities are cold, and femoral pulses are absent. The part of the vascular system that is affected in
this disorder is derived from which of the following embryologic structures?
A. Bulbus cordis
B. Ductus arteriosus
C. Left horn of sinus venosus
D. Right common cardinal vein
E. Right horn of sinus venosus
F. Third, fourth, and sixth aortic arches
Explanation:
The correct answer is F. This patient has coarctation of the aorta (constriction of the
ascending aorta), which is suggested by a midsystolic murmur over the anterior part of the
chest and back, hypertension in the upper extremities, and absent or delayed pulsations in the
femoral arteries. The upper extremities and thorax may be more developed than the lower
extremities. Patients with coarctation of the aorta may experience symptoms such as cold
extremities as a result of tissue ischemia.The truncus arteriosus gives rise to the proximal
portions of the ascending aorta and the pulmonary trunk. The third, 4th, and 6th aortic arches
and the right and left dorsal aortae contribute to the remainder of the aorta.
The bulbus cordis (choice A) gives rise to the right ventricle and the aortic outflow tract.
98% of cases of coarctation of the aorta take place immediately distal to the offshoot of the
left subclavian artery, close to the junction of the ductus arteriosus (choice B) with the aorta.
The left horn of the sinus venosus (choice C) gives rise to the coronary sinus.
The right common cardinal vein (choice D) gives rise to the superior vena cava.
The right horn of the sinus venosus (choice E) gives rise to the smooth part of the right atrium.
ok, i think these are 50+ que's.....GL.