Transcript USMLE MAY 14

1.When examining a histological section of a
normal ovary, a technician notices an oocyte
surrounded by several layers of follicular cells.
A small antrum is present. Which of the
following is the correct term for the entire
structure, composed of the oocyte, follicular
cells, and antrum?
A. Corpus luteum
B. Graafian follicle
C. Primary follicle
D. Primordial follicle
E. Secondary follicle
The correct answer is E.
Follicles in different stages of
maturation have different
appearances.
The most primitive follicles, primordial
follicles (choice D), are inactive
reserve follicles that contain primary
oocytes
(arrested in prophase of
first meiotic division) surrounded by a
single layer of flattened follicular cells.
Primary follicles (choice C), the next
stage, are slightly larger and contain
a central oocyte surrounded by one or
several cuboidal follicular cells.
When several small spaces in the
follicular mass fuse to form the
antrum (follicular cavity), the follicle is
termed a secondary follicle (choice
E).
The secondary follicles
continue to enlarge, and
develop a more complex
structure that includes
cumulus oophorus, corona
radiata, theca interna,
theca externa, and zona
pellucida.
The Graafian follicle (choice B) is the
mature form of the follicle,which
extends through the entire cortex and
bulges out at the ovarian surface.
After it ruptures and releases the
ovum, the corpus luteum (choice
A)develops as the cells of the follicle
and the theca interna cells enlarge,
become epithelioid, and secrete
estrogen.
The granulosa lutein cells contain
yellow pigment and secrete
progesterone. If pregnancy does
not occur, the corpus luteum
eventually degenerates; if
pregnancy occurs, it is maintained
throughout the pregnancy.
Zona pellucida
antrum
Granulosa
cells
Theca folliculi
follicles
2.The mother of a 7-year-old boy expresses
concern to her pediatrician that her son
has developed an irritated swelling on the
side of his neck. Palpation reveals a small,
mucus-secreting cyst along the anterior
border of the sternocleidomastoid muscle.
This branchial sinus, or external cervical
fistula, is the result of incomplete
obliteration of which of the following
developmental structures?
A. First pharyngeal arch
B. First pharyngeal cleft
C. First pharyngeal pouch
D. Second pharyngeal arch
E. Second pharyngeal cleft
F. Second pharyngeal pouch
• The correct answer is E.
• In normal development of the neck, the second
pharyngeal cleft grows inferiorly to fuse with the cardiac
ridge, thereby obliterating clefts 2-4. The cervical sinus
formed by the fusion of these tissue masses is normally
a transient structure, but may persist as a cyst, external
fistula, or internal fistula.
The first pharyngeal arch (choice A) gives rise to
various bony, muscular and ligamentous structures: the
malleus and incus; the muscles of mastication, plus the
tensor tympani, tensor veli palatini, mylohyoid and
anterior digastric; the sphenomandibular ligament and
anterior malleolar ligament.
• The first pharyngeal cleft (choice B) is retained
as the external auditory meatus and canal.
The first pharyngeal pouch (choice C) gives rise
to the tympanic cavity and auditory tube.
• The second pharyngeal arch (choice D), like the
first, gives rise to bones, muscles and ligaments:
the stapes and lesser horn and upper part of the
body of the hyoid bone; the muscles of facial
expression, plus the stapedius, stylohyoid and
posterior digastric; the stylohyoid ligament.
The second pharyngeal pouch (choice F) becomes
the tonsillar bed.
3.A newborn child has an abnormally formed
mandible, ears, and palate. He is diagnosed with
mandibulofacial dysostosis.
This syndrome is due to abnormal development of
which of the following structures?
A. First pharyngeal arch
B. First pharyngeal cleft
C. Second pharyngeal arch
D. Second pharyngeal cleft
E. Third pharyngeal cleft
F. Third pharyngeal pouch
• Explanation:
• The correct answer is A. The pharyngeal arches
are outpouchings of tissue visible on the
external neck of the embryo. They are separated
by the pharyngeal clefts (each one caudal to its
arch).
• The pharyngeal pouches are the outpouchings
of the pharynx visible inside the embryo that
correspond to the arches.
• The defect described is due to abnormal
development of the derivatives of the first
pharyngeal arch.
• It is thought that the initial defect may be
insufficient migration of neural crest cells.
4.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
• 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.
5.A 17-year-old male is examined by a
physician, who notes a mass at the back
of the young man's tongue.
The physician biopsies the mass, and the
pathology report comes back with a
diagnosis of normal thyroid tissue.
The occasional presence of such tissue at
the back of the tongue is related to the
embryonic origin of the thyroid near which
of the following structures?
A. First pharyngeal pouch
B. Foramen cecum
C. Nasolacrimal duct
D. Second pharyngeal arch
E. Third pharyngeal
pouch
•
The correct answer is B. The thyroid gland originates as
a mass of endodermal tissue near the foramen
cecum, which is near the tuberculum impar (which
becomes the central part of the tongue).
• During development, the thyroid descends in front of the
pharynx, maintaining a connection to the tongue via the
thyroglossal duct. Usually, the thyroglossal duct
disappears.
Uncommonly, residual ectopic thyroid tissue can be left
anywhere along the path, including at the back of the
tongue. (In rare patients, all of the thyroid tissue
remains at this site, forming a mass that should not be
excised, for obvious reasons!)
• The first pharyngeal pouch (choice A)
develops into the middle ear and
eustachian tube.
• The nasolacrimal ducts (choice C) connect
the eyes to the mouth.
• The second pharyngeal arch (choice D)
develops into many muscles of the face
and styloid process of the temporal bone.
• The third pharyngeal pouch (choice E)
develops into the thymus and inferior
glands.
6.Which of the following is not a foregut
derivative, but is supplied by an artery of
the foregut?
A. Liver
B. Lungs
C. Pancreas
D. Salivary glands
E. Spleen
• Explanation:
• The correct answer is E. Derivatives of the
foregut include all of the epithelial-lined organs
that attach to or include the mouth, esophagus,
stomach, and duodenum: oral cavity, pharynx,
tongue, tonsils, salivary glands (choice D), lower
respiratory system (choice B), esophagus,
stomach, duodenum proximal to the opening of
the bile duct, liver (choice A), gallbladder, and
pancreas (choice C). With the exception of the
pharynx,
• respiratory tract, and most of the esophagus,
these organs are supplied by the celiac artery.
• The celiac artery, via its splenic branch,
also supplies the spleen, which is not
classified as a foregut derivative, because
it develops from a mass of mesenchymal
cells (with no foregut epithelium) located
between the layers of the dorsal
mesogastrium.
7.A newborn infant has some of its
abdominal viscera protruding through a
defect in the abdominal wall. Which of the
following is the likely cause of this defect?
A. Failure of the intestinal loop to retract
from the umbilical cord
B. Failure of the yolk stalk to degenerate
C. Failure of peritoneal fusion
D. Incomplete fusion of the lateral body
folds
E. Umbilical herniation
• The correct answer is D.
During the fourth week of development, the
lateral body folds move ventrally and fuse
in the midline to form the anterior body
wall.
Incomplete fusion results in a defect that
allows abdominal viscera to protrude from
the abdominal cavity, a condition known
as gastroschisis.
• During development, the midgut normally
herniates into the umbilical cord and then
subsequently retracts into the abdominal cavity.
Failure of the intestinal loop to retract from the
umbilical cord (choice A) results in
omphalocele.
Failure of the yolk stalk to degenerate (choice B)
results in an ileal (Meckel's) diverticulum or a
vitelline fistula or cyst. In the early embryo, the
gut tube is connected to the yolk sac by a
narrow connection known as the yolk stalk.
Normally, this connection degenerates.
• During development, certain
peritoneal organs fuse with the
posterior abdominal wall to become
secondarily retroperitoneal.
• Failure of this peritoneal fusion
(choice C) will result in certain organs
that are normally immobile being
mobile (e.g., mobile cecum).
•
Umbilical herniation (choice E)
results from abdominal viscera
protruding through a weakness in
the abdominal wall after
development.
• Such protrusions are covered by
subcutaneous fascia and skin,
distinguishing them from
gastroschisis.
8.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. Ileu
E. Jejunum
• 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.
9.A neonate with multiple congenital anomalies is
found to have a complete absence of one
kidney. Other genitourinary structures are intact.
This is most likely the result of which of the
following?
A. Absence of the ureteric bud
B. Complete division of the ureteric bud
C. Crossed renal ectopia
D. Double ureteric buds
E. Incomplete division of ureteric bud
• The correct answer is A. Unilateral renal
agenesis is a common (1 in 1000 births)
congenital anomaly that
may be completely asymptomatic, or may,
as in this case, be identified during
evaluation of other congenital
anomalies. A single umbilical artery may
also be present, and may offer a clinical
clue.
Unilateral renal agenesis is usually the
result of a failure of the ureteric bud to
develop.
Complete division of a ureteric bud (choice
B) produces a bifid ureter with a double
kidney on one side.
Crossed renal ectopia (choice C) produces
fused kidneys on one side.
Double ureteric buds (choice D) produces a
supernumerary kidney.
Incomplete division of a ureteric bud
(choice E) produces a bifid ureter with a
partially divided kidney.