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usmle step 1 embryology-u world
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1. A male infant born to a 33-year-old African American femalefails to pass meconium during the first 24 hours after his birth.
On physical examination, a "dimple" is found instead of an
anal opening in the perineum. Which of the following
additional defects are most likely to be found in this patient?
A. Tracheoesophageal fistula
B. Vertebral abnormalities
C. Urinary tract defects
D. Cardiac abnormalities
E. Ileal atresia
F. Biliary atresia
Answer: C
Explanation:
The anal canal extends from the anorectal junction (perineal flexure) to
the perianal skin (anal verge). It is divided into upper and lower anal
canal. The upper anal canal (above the pectinate line) is formed from the
hindgut. The lower part of the anal canal is derived from the invagination
of surface ectoderm. The junction between these canals is closed during
embryonic life by an anal membrane that is located at the level of the
pectinate line.
The term "imperforate anus" covers a spectrum of disorders associated
with abnormal development of anorectal structures. Absence of the anal
opening is most often associated with urorectal, uterovesical, or
urogenital fistulas. When a fistula is present, meconium may discharge
from the urethra or the vagina.
Imperforate anus is most commonly associated with genitourinary tract
malformations. These partnered malformations include renal agenesis,
hypospadias, epispadias, and bladder exstrophy. Such abnormalities are
reported in up to 50% of patients with imperforate anus.
,(Choices A, B and D) Other congenital anomalies also occur in
association with imperforate anus. The acronym VACTERL describes
these anomalies: vertebral defects, anal atresia, cardiac anomalies,
tracheoesophageal fistula, esophageal atresia, ren:al anomalies, and
limb anomalies. VACTERL syndrome is much less common than
isolated urogenital anomalies.
(Choices E and F) Neither ileal atresia nor biliary atresia are associated
with imperforate anus.
2. A 5-month-old Caucasian male is brought to your office by his mother
who noticed right-sided scrotal enlargement. On ultrasonography, the
enlargement is found to be a fluid collection around the right testis. This
patient's condition has pathophysiology most similar to which of the
following?
A. Femoral hernia
B. Direct inguinal hernia
C. Indirect inguinal hernia
D. Acute orchitis
E. Acute testicular torsion
F. Hypospadia
Answer: C
Explanation:
In early foetal life, the testes are located in the peritoneal cavity and
descend into the scrotum before birth. During the descent, they are
accompanied by an invagination of the peritoneum called the processus
vagina/is. This structure follows the testes into the scrotum and
obliterates after testicular descent is complete. Remnants of the
processus vaginalis form the tunica vaginalis of the testes.
,Failure of obliteration of the processus vaginalis leads to a persistent
connection between the scrotum and the peritoneal cavity through the
inguinal canal. If the opening is small and allows for fluid leakage
only,hydrocele occurs. Diagnosis is by transillumination of the scrotum
and scrotal ultrasound, which reveal fluid (only) in the tunica vaginalis
sac.
If the communication between the peritoneal cavity and the scrotum
allow s for the passage of abdominal organs, an indirect inguinal hernia
develops. A bulge on the groyne that increases during straining is the
typical manifestation .
(Choice A) Femoral hernias occur due to protrusion of abdominal
contents through the femoral canal.
(Choice B) Direct inguinal hernias form due to protrusion of the
abdominal contents through the Hesselbach triangle on the anterior
abdominal wall. The processus vaginalis does not participate in
information of this type of hernia.
(Choice D) Acute orchitis is a non-specific inflammation of the testes. In
young adults and adolescents, it is commonly caused by C. trachomatis
and N. gonorrhoeae. In older patients, E. coli is the most common
causative agent.
(Choice E) Testicular torsion is a rotation of the testes around the
spermatic cord. Trauma or a congenitally-horizontal positioning of the
testes ("bell clapper deformity") facilitates torsion.
(Choice F) Hypospadias occurs due to incomplete closure of the
urethral folds. As a result, the urethral opening is located on the ventral
surface of the penis.
, 3. A 22-year-old woman at 14 weeks gestation comes to the physician
for a prenatal visit. She reports feeling well with the exception of some
mild fatigue. Her pregnancy has been uncomplicated to date. On
physical examination, the patient's abdomen appears larger than would
be expected at 14 weeks. An obstetrical ultrasound reveals twins, a male
and a female. Which of the following types of twin placentation is most
likely present?
A. dichorionic/diamniotic
B. dichorionic/monoamniotic
C. monochorionic/diamniotic
D. monochorionic/monoamniotic
E. monochorionic/monoamniotic conjoined
Answer: A
Explanation:
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