Embryologic Development and Defects Practice Questions
20 free Embryologic Development and Defects practice questions for the USMLE Step 1. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
A newborn has a midline abdominal defect with herniation of abdominal organs covered only by a thin membrane. Which embryologic failure is most likely responsible?
- A Failure of lateral folds to fuse
- B Persistence of the vitelline duct
- C Incomplete return of midgut to the abdomen
- D Incomplete fusion of urorectal septum
Correct answer: Incomplete return of midgut to the abdomen
Omphalocele results from failure of the herniated midgut to return to the abdominal cavity. The organs remain covered by a peritoneal and amniotic membrane, unlike gastroschisis, which has no covering and arises from lateral body-fold failure.
A 2-week-old infant has meconium discharge from the umbilicus. This finding is most consistent with persistence of which structure?
- A Allantois
- B Vitelline duct
- C Urachus
- D Ductus venosus
Correct answer: Vitelline duct
Persistence of the vitelline duct leads to a patent omphalomesenteric duct, causing stool discharge from the umbilicus. It may also cause Meckel diverticulum.
A congenital diaphragmatic hernia results from failed closure of which embryologic structure?
- A Septum transversum
- B Pleuroperitoneal membrane
- C Dorsal mesentery of esophagus
- D Body wall musculature
Correct answer: Pleuroperitoneal membrane
Most congenital diaphragmatic hernias arise from failure of the pleuroperitoneal membranes to close the pericardioperitoneal canals. This allows abdominal organs to herniate into the thorax.
A fetus has polyhydramnios. Ultrasound reveals esophageal atresia. This condition is most directly caused by abnormal development of which structure?
- A Tracheoesophageal septum
- B Laryngotracheal groove
- C Bronchial buds
- D Pharyngeal pouches
Correct answer: Tracheoesophageal septum
Abnormal partitioning of the esophagus and trachea by the tracheoesophageal septum causes esophageal atresia and tracheoesophageal fistula. Polyhydramnios results from impaired fetal swallowing.
A neonate with cyanosis worsens with feeding and improves with crying. Which embryologic defect most likely explains this?
- A Choanal atresia
- B Cleft palate
- C Nasal septal deviation
- D Laryngeal atresia
Correct answer: Choanal atresia
Choanal atresia results from failure of the oronasal membrane to break down. Newborns are obligate nasal breathers, so feeding-related cyanosis is characteristic.
A neonate with Down syndrome has bilious vomiting. Imaging shows a 'double-bubble' sign. Which embryologic event is disrupted?
- A Recanalization of the duodenum
- B Rotation of the midgut
- C Partitioning of the cloaca
- D Fusion of pancreatic buds
Correct answer: Recanalization of the duodenum
Duodenal atresia results from failure of recanalization of the duodenum. It is strongly associated with Down syndrome and shows a 'double-bubble' sign on imaging.
A newborn has urine leaking from the umbilicus. This condition results from persistence of which fetal structure?
- A Vitelline artery
- B Urachus
- C Allantois
- D Mesonephric duct
Correct answer: Urachus
A patent urachus (remnant of the allantois) causes urine discharge from the umbilicus. It normally obliterates to form the median umbilical ligament.
A neonate has a systolic murmur and imaging reveals failure of the aorticopulmonary septum to develop properly. Which congenital defect is likely present?
- A Tetralogy of Fallot
- B Persistent truncus arteriosus
- C Transposition of the great arteries
- D Coarctation of the aorta
Correct answer: Persistent truncus arteriosus
Persistent truncus arteriosus results from failure of neural crest-derived conotruncal ridges to fuse. This forms a single outflow tract.
A fetus is diagnosed with congenital aganglionic megacolon (Hirschsprung disease). This disorder is caused by failure of migration of which cells?
- A Mesodermal myoblasts
- B Neural crest cells
- C Endodermal epithelial cells
- D Mesonephric duct cells
Correct answer: Neural crest cells
Neural crest cells migrate to form the enteric nervous system. Failure of migration causes aganglionic distal colon and functional obstruction.
A newborn has a left-sided diaphragmatic hernia resulting in respiratory distress. Which fetal structure failed to develop normally?
- A Septum transversum
- B Pleuroperitoneal folds
- C Dorsal mesogastrium
- D Laryngotracheal diverticulum
Correct answer: Pleuroperitoneal folds
Failure of the pleuroperitoneal folds to close allows abdominal contents to enter the thorax. Left-sided defects are more common due to the weaker left fusion.
A neonate has a midline cleft lip. This defect results from failed fusion between which structures?
- A Medial nasal processes
- B Medial and lateral nasal processes
- C Maxillary and medial nasal prominences
- D Mandibular processes
Correct answer: Medial nasal processes
Midline cleft lip results from failure of the medial nasal prominences to fuse. Lateral cleft lip involves the maxillary prominence instead.
A newborn with spina bifida occulta shows a tuft of hair over the lumbar spine. Which embryologic structure failed to close?
- A Neural tube
- B Somites
- C Notochord
- D Lateral body folds
Correct answer: Neural tube
Spina bifida results from abnormal closure of the posterior neuropore. Occulta is the mildest form and is often associated with a skin marker like a hair tuft.
A child presents with a horseshoe kidney. This congenital anomaly is due to fusion of the kidneys at which structure?
- A Upper poles
- B Lower poles
- C Renal hilum
- D Renal medulla
Correct answer: Lower poles
Horseshoe kidneys fuse at the lower poles. As they ascend, they become trapped under the inferior mesenteric artery.
A neonate exhibits a holosystolic murmur due to a ventricular septal defect. This defect is most commonly due to failure of formation of which septum?
- A Membranous interventricular septum
- B Muscular interventricular septum
- C Endocardial cushion septum
- D Septum transversum
Correct answer: Membranous interventricular septum
Most VSDs occur in the membranous part of the IV septum, derived from endocardial cushions. These defects are the most common congenital heart defects.
A newborn has intestinal obstruction due to malrotation of the midgut. Which embryologic event normally occurs at 10 weeks gestation?
- A Recanalization of the esophagus
- B Return of the midgut to the abdomen
- C Formation of the anal membrane
- D Fusion of the pancreatic ducts
Correct answer: Return of the midgut to the abdomen
The midgut returns to the abdominal cavity at week 10 after physiological herniation. Failure can lead to volvulus and obstruction.
A neonate presents with a persistent cervical sinus. This defect results from incomplete involution of which structure?
- A Pharyngeal grooves
- B Pharyngeal pouches
- C Pharyngeal arches
- D Branchial membranes
Correct answer: Pharyngeal grooves
Persistent cervical sinus results from failure of the second pharyngeal arch to grow over the third and fourth pharyngeal grooves. It may form a branchial fistula or cyst.
A newborn presents with Potter sequence. Which embryologic abnormality most likely caused this condition?
- A Failure of ureteric bud to develop
- B Incomplete recanalization of urethra
- C Persistent cloacal membrane
- D Failure of the Mullerian ducts to fuse
Correct answer: Failure of ureteric bud to develop
Bilateral renal agenesis results from failure of the ureteric bud to develop and induces oligohydramnios. This causes the characteristic features of Potter sequence.
A fetus shows intestinal contents within the umbilical cord covered by peritoneum. This condition is caused by which embryologic failure?
- A Failure of the lateral folds to fuse
- B Persistent physiologic midgut herniation
- C Failure of recanalization of the duodenum
- D Persistence of the urachus
Correct answer: Persistent physiologic midgut herniation
Omphalocele occurs when physiologic herniation of the midgut fails to return to the abdominal cavity. The herniated organs remain covered by peritoneum.
A neonate is diagnosed with annular pancreas. This condition is most often caused by abnormal migration of which structure?
- A Dorsal pancreatic bud
- B Ventral pancreatic bud
- C Hepatic diverticulum
- D Mesonephric ducts
Correct answer: Ventral pancreatic bud
Annular pancreas occurs when the ventral pancreatic bud splits and encircles the duodenum. This can cause duodenal obstruction.
A newborn has imperforate anus. This condition results from abnormal development of which embryologic structure?
- A Urogenital sinus
- B Cloacal membrane
- C Allantois
- D Vitelline duct
Correct answer: Cloacal membrane
Imperforate anus occurs when the anal membrane fails to perforate. This is due to abnormal development of the cloacal membrane and urorectal septum.