Embryology — MCQs

Q: What is the pattern of inheritance in neural tube defects?

Multifactorial inheritance. ***Multifactorial inheritance*** - Neural tube defects (NTDs) are a classic example of **multifactorial inheritance**, meaning they result from a combination of **genetic predispositions** and **environmental factors**. - Risk is influenced by multiple genes, and environmental factors like **folate deficiency** play a significant role. *Autosomal recessive* - This pattern involves two copies of an altered gene to cause disease, typically resulting in a **25% recurrence risk** for siblings. - While some rare isolated NTDs might have an autosomal recessive component, the general presentation of NTDs does not fit this classic mendelian pattern. *X-linked dominant* - Involves genes on the **X chromosome** where one altered copy is sufficient to cause disease; affected fathers pass it to all daughters, but no sons. - This inheritance pattern is very rare for NTDs and would present with a distinct sex-linked pattern of affected individuals. *Autosomal dominant* - Requires only one copy of an altered gene to cause disease, leading to a **50% recurrence risk** for offspring. - While some syndromes associated with NTDs can be autosomal dominant, the primary mechanism for isolated NTDs is not solely due to a single dominant gene. *X-linked recessive* - Involves genes on the **X chromosome** where two altered copies are needed in females, but only one in males; typically affects males predominantly. - This inheritance pattern does not account for the observed familial clustering and environmental contribution seen in NTDs.

Q: The covering of an omphalocele is derived from which of the following layers?

Amnion. ***Amnion*** - An **omphalocele** is a congenital abdominal wall defect where abdominal contents protrude through the umbilical ring, covered by a sac derived from the **amnion** and peritoneum. - The covering of an omphalocele defect is an intact peritoneal sac that is covered externally by **amnion**. *Chorion* - The **chorion** is the outermost membrane surrounding an embryo, providing protection and nourishment, but it does not form the covering of an omphalocele. - It works in conjunction with the decidua to form the **placenta** and has finger-like projections called villi on its outer surface. *Mesoderm* - The **mesoderm** is one of the three primary germ layers in embryonic development, giving rise to connective tissue, muscle, and blood cells. - **Fetal skin**, not the omphalocele covering, develops from the mesoderm and ectoderm. *Endoderm* - The **endoderm** is the innermost of the three primary germ layers, giving rise to the lining of the digestive tract and respiratory system. - The omphalocele covering is derived from the amnion, not the endoderm, which is involved in forming internal organs. *Ectoderm* - The **ectoderm** is the outermost of the three primary germ layers, giving rise to the nervous system, skin epidermis, and sensory organs. - While ectoderm contributes to skin development, the omphalocele sac is specifically covered by amnion, not ectodermal derivatives.

Q: Identify the uterus shown in the image?

Bicornuate. ***Bicornuate*** - The image displays a uterus with two distinct uterine horns that are partially fused at the fundus, creating a **heart-shaped external indentation**. - This morphology is characteristic of a **bicornuate uterus**, which results from incomplete fusion of the paramesonephric (Müllerian) ducts. *Didelphys* - A **uterus didelphys** would show two completely separate uteri, each with its own cervix and often a separate vagina. - The image clearly shows a single cervix and a single vaginal canal, with only the upper uterine body being divided. *Arcuate* - An **arcuate uterus** has a small, concave indentation at the fundus, but the overall shape is typically that of a normal uterus with a mild deformity. - The image shows a much more significant division of the uterine cavity than seen in an arcuate uterus. *Septate* - A **septate uterus** has a normal external contour but contains an internal septum that divides the uterine cavity into two parts. - The image demonstrates both internal and external division with a heart-shaped fundal indentation, which distinguishes bicornuate from septate uterus. *Unicornuate* - A **unicornuate uterus** results from failure of one Müllerian duct to develop, creating a single elongated uterine horn. - The image shows two uterine horns, not a single horn, ruling out this diagnosis.

Q: Scientists are studying human lung development by trying to identify which proteins and signaling factors trigger lung bud division and bronchiole branching. Their main focus is particularly around the 20th week of gestation, during which terminal bronchioles branch into respiratory bronchioles and further into alveolar ducts. Which of the following phases of embryonic lung development is the stage in which the scientists are interested in studying?

Canalicular. ***Canalicular*** - The **canalicular stage**, occurring from weeks 16 to 26, is characterized by the formation of **respiratory bronchioles**, alveolar ducts, and the beginning of vascularization. - This stage is crucial for the development of blood-air barrier precursors, making it relevant for studying branching and differentiation into gas exchange structures. *Alveolar* - The **alveolar stage** begins much later (late fetal period, 32 weeks, continuing for several years postpartum) and primarily involves the maturation of **alveoli** and increase in their numbers. - While essential for complete lung function, it does not describe the initial branching and differentiation of bronchioles that occurs around 20 weeks. *Pseudoglandular* - The **pseudoglandular stage**, from weeks 6 to 16, involves the formation of the major conducting airways (bronchi and terminal bronchioles) but not yet respiratory bronchioles or alveolar ducts. - The description of terminal bronchioles branching into respiratory bronchioles places it beyond this stage. *Saccular* - The **saccular stage**, from weeks 26 to 32, is marked by the formation of saccules (primitive alveoli) and further thinning of the blood-air barrier. - While it involves growth of the respiratory tree, the detailed branching into respiratory bronchioles and alveolar ducts is primarily a feature of the preceding canalicular stage. *Embryonic* - The **embryonic stage**, from weeks 3 to 6, is the earliest phase where the lung bud emerges from the foregut and begins to divide into main bronchi. - This stage is too early for the significant branching described, which involves terminal and respiratory bronchioles.

Q: During the obstetric clerkship, the doctor is observing a 3rd-year resident assist a delivery. The patient only had 1 prenatal ultrasound that reported a male fetus. The delivery progresses without complications. The pediatrician-in-charge of the newborn notices a short, broad, upturned penis with an orifice in its dorsal aspect, and both testicles are present in the scrotum. Both the attending and PGY-3 resident immediately recognize the condition. Which of the following female anatomical structures is derived from the embryonic structure affected in this patient?

Clitoris. ***Clitoris*** The newborn has **epispadias**, a congenital malformation characterized by the urethral opening on the dorsal (upper) surface of the penis. This condition results from abnormal development of the **genital tubercle**, which forms during weeks 4-7 of embryonic development. In males, the genital tubercle normally differentiates into the glans and shaft of the penis. In females, this same embryonic structure gives rise to the **clitoris**. Since the affected structure in this patient is the genital tubercle, the corresponding female anatomical structure is the clitoris. *Labia minora* This structure is derived from the **urogenital folds**, not the genital tubercle. In males, the urogenital folds fuse to form the ventral aspect of the penis and enclose the penile urethra. The patient's epispadias affects the genital tubercle, not the urogenital folds. *Labia majora* This structure is derived from the **labioscrotal swellings**, which in males form the scrotum. The question states both testicles are present in the scrotum, indicating normal labioscrotal development. The malformation involves the genital tubercle, not the labioscrotal swellings. *Vestibule* The vestibule is derived from the **urogenital sinus**, not the genital tubercle. In males, the urogenital sinus contributes to the prostatic and membranous urethra, as well as the bulbourethral glands. The dorsal penile defect in epispadias specifically affects genital tubercle development. *Bartholin glands* These greater vestibular glands are derived from the **urogenital sinus**, which in males gives rise to the bulbourethral (Cowper's) glands. The patient's condition involves the genital tubercle, not structures derived from the urogenital sinus.

Q: A 36-year-old man comes to the physician for a follow-up examination. Two weeks ago, he was diagnosed with an ischemic stroke of the right middle cerebral artery. He was treated with thrombolytics and does not have any residual symptoms. His pulse is 82/min and regular. Cardiovascular examination shows no abnormalities. Echocardiography shows a reproducible, transient, low-volume, right-to-left shunt through the atrial septum during coughing. Which of the following conditions is caused by failure of an embryologic process similar to that responsible for this patient's heart condition?

Hypospadias. ***Hypospadias*** - The patient's presentation with a **transient right-to-left shunt** through the atrial septum during coughing, especially after an ischemic stroke, suggests a **patent foramen ovale (PFO)**. A PFO results from the failed fusion of the **septum primum** and **septum secundum** postnatally. - Hypospadias is a developmental anomaly where the urethral opening is on the ventral side of the penis, resulting from the **incomplete fusion of the urethral folds** during embryogenesis. Both PFO and hypospadias arise from failures of fusion of embryonic structures. *Transposition of the great vessels* - This condition is caused by a failure of the **aorticopulmonary septum** to spiral during septation of the truncus arteriosus, leading to the aorta originating from the right ventricle and the pulmonary artery from the left ventricle. - This is a defect in **septation and spiraling**, not a failure of fusion of distinct embryonic structures. *Hirschsprung disease* - This is characterized by the **absence of ganglion cells** (Meissner and Auerbach plexuses) in the distal colon, due to the failure of **neural crest cell migration** to the affected bowel segments. - This condition is a defect in cell migration, distinct from a failure of embryonic structure fusion. *Midgut volvulus* - This occurs due to **malrotation of the intestines** during embryonic development, leading to an abnormally positioned small bowel that is prone to twisting around the superior mesenteric artery. - This is a defect in **intestinal rotation and fixation**, rather than a failure of fusion. *Thyroglossal duct cyst* - This results from the **failure of complete obliteration of the thyroglossal duct**, an embryonic remnant formed during the descent of the thyroid gland from the tongue base to its final position in the neck. - While it involves an embryonic remnant, it is a failure of **obliteration**, not fusion, of two separate structures.

Q: A newborn boy born vaginally to a healthy 37-year-old G3P1 from a pregnancy complicated by hydramnios fails to pass meconium after 24 hours of life. The vital signs are within normal limits for his age. The abdomen is distended, the anus is patent, and the rectal examination reveals pale mucous with non-pigmented meconium. Based on a barium enema, the boy is diagnosed with sigmoid colonic atresia. Disruption of which structure during fetal development could lead to this anomaly?

Inferior mesenteric artery. ***Inferior mesenteric artery*** - **Sigmoid colonic atresia**, as observed in this case, results from an ischemic event affecting the segment of the bowel supplied by the **inferior mesenteric artery** during fetal development. - Interruption of blood flow to this region can lead to subsequent **atresia** as the affected part of the intestine necroses and is reabsorbed. *Superior mesenteric artery* - The **superior mesenteric artery** primarily supplies the midgut structures, including the small intestine and parts of the large intestine up to the transverse colon. - Disruption of the superior mesenteric artery would typically lead to atresias higher up in the **gastrointestinal tract**, such as jejunal or ileal atresias, not sigmoid colonic atresia. *Vitelline duct* - The **vitelline duct** (also known as the omphalomesenteric duct) connects the midgut to the yolk sac during early fetal development. - Persistent patency or partial obliteration of the vitelline duct can lead to anomalies like **Meckel's diverticulum** or vitelline cysts, which are distinct from colonic atresia. *Cloaca* - The **cloaca** is a common cavity for the digestive, urinary, and reproductive tracts during early embryonic development. - Defects in cloacal development lead to complex malformations involving these systems, such as **imperforate anus** or persistent cloaca, rather than isolated colonic atresia with a patent anus. *Celiac artery* - The **celiac artery** supplies the foregut structures, including the stomach, duodenum, liver, and spleen. - Disruption of the celiac artery during fetal development would result in malformations of these upper gastrointestinal organs, not the sigmoid colon.

Q: A research team is studying certain congenital anomalies of the respiratory tract. The method consists of marking a certain germinal layer with an isotope, following its development stages in chicken embryos, and finally analyzing the specimen. A given specimen of tissue is presented in the exhibit. Which of the following germinal structures most likely gave rise to the epithelial lining of this specimen?

Endoderm. ***Endoderm*** - The **epithelial lining** of the entire respiratory tract, including the larynx, trachea, bronchi, and lungs, is derived from the **endoderm**. - The **laryngotracheal groove** develops from the ventral wall of the primitive foregut, which is endodermal in origin, further differentiating into the respiratory tree. *Ectoderm* - The **ectoderm** primarily forms the epidermis, hair, nails, and the nervous system (brain and spinal cord). - It does not contribute to the internal epithelial lining of the respiratory tract. *Neural crest* - **Neural crest cells** contribute to a wide variety of structures, including components of the peripheral nervous system, head mesenchyme, and melanocytes. - They are not involved in forming the epithelial lining of the respiratory system. *Mesoderm* - The **mesoderm** forms the connective tissue, cartilage, and muscle components of the respiratory tract, such as the smooth muscle and cartilage rings of the trachea and bronchi, and the visceral pleura. - However, it does not form the epithelial lining itself. *Surface ectoderm* - **Surface ectoderm** specifically gives rise to the epidermis, hair, nails, and glands of the skin, as well as the oral cavity epithelium. - It does not contribute to the internal epithelial structures of the respiratory tract.

Scientists are studying human lung development by trying to identify which proteins and signaling factors trigger lung bud division and bronchiole branching. Their main focus is particularly around the 20th week of gestation, during which terminal bronchioles branch into respiratory bronchioles and further into alveolar ducts. Which of the following phases of embryonic lung development is the stage in which the scientists are interested in studying?

During the obstetric clerkship, the doctor is observing a 3rd-year resident assist a delivery. The patient only had 1 prenatal ultrasound that reported a male fetus. The delivery progresses without complications. The pediatrician-in-charge of the newborn notices a short, broad, upturned penis with an orifice in its dorsal aspect, and both testicles are present in the scrotum. Both the attending and PGY-3 resident immediately recognize the condition. Which of the following female anatomical structures is derived from the embryonic structure affected in this patient?

A primigravid 28-year-old woman delivers a 38-week-old male infant via spontaneous vaginal delivery. She had no prenatal care during her pregnancy. At birth the infant has underdeveloped hands and radiograph reveals missing phalanges in the thumbs. Examination of the buttocks reveals a missing anus. Further work-up reveals flow between the two ventricles on echocardiography and a single kidney on preliminary abdominal ultrasound. The infant also has difficulty feeding that results in coughing and apnea. Which of the following tissues was most likely affected during embryologic development?

A 36-year-old man comes to the physician for a follow-up examination. Two weeks ago, he was diagnosed with an ischemic stroke of the right middle cerebral artery. He was treated with thrombolytics and does not have any residual symptoms. His pulse is 82/min and regular. Cardiovascular examination shows no abnormalities. Echocardiography shows a reproducible, transient, low-volume, right-to-left shunt through the atrial septum during coughing. Which of the following conditions is caused by failure of an embryologic process similar to that responsible for this patient's heart condition?

A newborn boy born vaginally to a healthy 37-year-old G3P1 from a pregnancy complicated by hydramnios fails to pass meconium after 24 hours of life. The vital signs are within normal limits for his age. The abdomen is distended, the anus is patent, and the rectal examination reveals pale mucous with non-pigmented meconium. Based on a barium enema, the boy is diagnosed with sigmoid colonic atresia. Disruption of which structure during fetal development could lead to this anomaly?

A research team is studying certain congenital anomalies of the respiratory tract. The method consists of marking a certain germinal layer with an isotope, following its development stages in chicken embryos, and finally analyzing the specimen. A given specimen of tissue is presented in the exhibit. Which of the following germinal structures most likely gave rise to the epithelial lining of this specimen?

Q10

A 26-year-old woman comes to the physician because she has not had a menstrual period for 5 weeks. Menarche was at the age of 14 years and menses occurred at regular 30-day intervals. She reports having unprotected sexual intercourse 3 weeks ago. A urine pregnancy test is positive. Which of the following best describes the stage of development of the embryo at this time?

Embryology US Medical PG Practice Questions and MCQs

Question 1: What is the pattern of inheritance in neural tube defects?

A. Multifactorial inheritance (Correct Answer)
B. Autosomal recessive
C. X-linked dominant
D. Autosomal dominant
E. X-linked recessive

Explanation: ***Multifactorial inheritance*** - Neural tube defects (NTDs) are a classic example of **multifactorial inheritance**, meaning they result from a combination of **genetic predispositions** and **environmental factors**. - Risk is influenced by multiple genes, and environmental factors like **folate deficiency** play a significant role. *Autosomal recessive* - This pattern involves two copies of an altered gene to cause disease, typically resulting in a **25% recurrence risk** for siblings. - While some rare isolated NTDs might have an autosomal recessive component, the general presentation of NTDs does not fit this classic mendelian pattern. *X-linked dominant* - Involves genes on the **X chromosome** where one altered copy is sufficient to cause disease; affected fathers pass it to all daughters, but no sons. - This inheritance pattern is very rare for NTDs and would present with a distinct sex-linked pattern of affected individuals. *Autosomal dominant* - Requires only one copy of an altered gene to cause disease, leading to a **50% recurrence risk** for offspring. - While some syndromes associated with NTDs can be autosomal dominant, the primary mechanism for isolated NTDs is not solely due to a single dominant gene. *X-linked recessive* - Involves genes on the **X chromosome** where two altered copies are needed in females, but only one in males; typically affects males predominantly. - This inheritance pattern does not account for the observed familial clustering and environmental contribution seen in NTDs.

Question 2: The covering of an omphalocele is derived from which of the following layers?

A. Amnion (Correct Answer)
B. Chorion
C. Mesoderm
D. Endoderm
E. Ectoderm

Explanation: ***Amnion*** - An **omphalocele** is a congenital abdominal wall defect where abdominal contents protrude through the umbilical ring, covered by a sac derived from the **amnion** and peritoneum. - The covering of an omphalocele defect is an intact peritoneal sac that is covered externally by **amnion**. *Chorion* - The **chorion** is the outermost membrane surrounding an embryo, providing protection and nourishment, but it does not form the covering of an omphalocele. - It works in conjunction with the decidua to form the **placenta** and has finger-like projections called villi on its outer surface. *Mesoderm* - The **mesoderm** is one of the three primary germ layers in embryonic development, giving rise to connective tissue, muscle, and blood cells. - **Fetal skin**, not the omphalocele covering, develops from the mesoderm and ectoderm. *Endoderm* - The **endoderm** is the innermost of the three primary germ layers, giving rise to the lining of the digestive tract and respiratory system. - The omphalocele covering is derived from the amnion, not the endoderm, which is involved in forming internal organs. *Ectoderm* - The **ectoderm** is the outermost of the three primary germ layers, giving rise to the nervous system, skin epidermis, and sensory organs. - While ectoderm contributes to skin development, the omphalocele sac is specifically covered by amnion, not ectodermal derivatives.

Question 3: Identify the uterus shown in the image?

A. Bicornuate (Correct Answer)
B. Didelphys
C. Arcuate
D. Septate
E. Unicornuate

Explanation: ***Bicornuate*** - The image displays a uterus with two distinct uterine horns that are partially fused at the fundus, creating a **heart-shaped external indentation**. - This morphology is characteristic of a **bicornuate uterus**, which results from incomplete fusion of the paramesonephric (Müllerian) ducts. *Didelphys* - A **uterus didelphys** would show two completely separate uteri, each with its own cervix and often a separate vagina. - The image clearly shows a single cervix and a single vaginal canal, with only the upper uterine body being divided. *Arcuate* - An **arcuate uterus** has a small, concave indentation at the fundus, but the overall shape is typically that of a normal uterus with a mild deformity. - The image shows a much more significant division of the uterine cavity than seen in an arcuate uterus. *Septate* - A **septate uterus** has a normal external contour but contains an internal septum that divides the uterine cavity into two parts. - The image demonstrates both internal and external division with a heart-shaped fundal indentation, which distinguishes bicornuate from septate uterus. *Unicornuate* - A **unicornuate uterus** results from failure of one Müllerian duct to develop, creating a single elongated uterine horn. - The image shows two uterine horns, not a single horn, ruling out this diagnosis.

Question 4: Scientists are studying human lung development by trying to identify which proteins and signaling factors trigger lung bud division and bronchiole branching. Their main focus is particularly around the 20th week of gestation, during which terminal bronchioles branch into respiratory bronchioles and further into alveolar ducts. Which of the following phases of embryonic lung development is the stage in which the scientists are interested in studying?

A. Canalicular (Correct Answer)
B. Alveolar
C. Pseudoglandular
D. Saccular
E. Embryonic

Explanation: ***Canalicular*** - The **canalicular stage**, occurring from weeks 16 to 26, is characterized by the formation of **respiratory bronchioles**, alveolar ducts, and the beginning of vascularization. - This stage is crucial for the development of blood-air barrier precursors, making it relevant for studying branching and differentiation into gas exchange structures. *Alveolar* - The **alveolar stage** begins much later (late fetal period, 32 weeks, continuing for several years postpartum) and primarily involves the maturation of **alveoli** and increase in their numbers. - While essential for complete lung function, it does not describe the initial branching and differentiation of bronchioles that occurs around 20 weeks. *Pseudoglandular* - The **pseudoglandular stage**, from weeks 6 to 16, involves the formation of the major conducting airways (bronchi and terminal bronchioles) but not yet respiratory bronchioles or alveolar ducts. - The description of terminal bronchioles branching into respiratory bronchioles places it beyond this stage. *Saccular* - The **saccular stage**, from weeks 26 to 32, is marked by the formation of saccules (primitive alveoli) and further thinning of the blood-air barrier. - While it involves growth of the respiratory tree, the detailed branching into respiratory bronchioles and alveolar ducts is primarily a feature of the preceding canalicular stage. *Embryonic* - The **embryonic stage**, from weeks 3 to 6, is the earliest phase where the lung bud emerges from the foregut and begins to divide into main bronchi. - This stage is too early for the significant branching described, which involves terminal and respiratory bronchioles.

Question 5: During the obstetric clerkship, the doctor is observing a 3rd-year resident assist a delivery. The patient only had 1 prenatal ultrasound that reported a male fetus. The delivery progresses without complications. The pediatrician-in-charge of the newborn notices a short, broad, upturned penis with an orifice in its dorsal aspect, and both testicles are present in the scrotum. Both the attending and PGY-3 resident immediately recognize the condition. Which of the following female anatomical structures is derived from the embryonic structure affected in this patient?

A. Clitoris (Correct Answer)
B. Labia minora
C. Labia majora
D. Vestibule
E. Bartholin glands

Explanation: ***Clitoris*** The newborn has **epispadias**, a congenital malformation characterized by the urethral opening on the dorsal (upper) surface of the penis. This condition results from abnormal development of the **genital tubercle**, which forms during weeks 4-7 of embryonic development. In males, the genital tubercle normally differentiates into the glans and shaft of the penis. In females, this same embryonic structure gives rise to the **clitoris**. Since the affected structure in this patient is the genital tubercle, the corresponding female anatomical structure is the clitoris. *Labia minora* This structure is derived from the **urogenital folds**, not the genital tubercle. In males, the urogenital folds fuse to form the ventral aspect of the penis and enclose the penile urethra. The patient's epispadias affects the genital tubercle, not the urogenital folds. *Labia majora* This structure is derived from the **labioscrotal swellings**, which in males form the scrotum. The question states both testicles are present in the scrotum, indicating normal labioscrotal development. The malformation involves the genital tubercle, not the labioscrotal swellings. *Vestibule* The vestibule is derived from the **urogenital sinus**, not the genital tubercle. In males, the urogenital sinus contributes to the prostatic and membranous urethra, as well as the bulbourethral glands. The dorsal penile defect in epispadias specifically affects genital tubercle development. *Bartholin glands* These greater vestibular glands are derived from the **urogenital sinus**, which in males gives rise to the bulbourethral (Cowper's) glands. The patient's condition involves the genital tubercle, not structures derived from the urogenital sinus.

Question 6: A primigravid 28-year-old woman delivers a 38-week-old male infant via spontaneous vaginal delivery. She had no prenatal care during her pregnancy. At birth the infant has underdeveloped hands and radiograph reveals missing phalanges in the thumbs. Examination of the buttocks reveals a missing anus. Further work-up reveals flow between the two ventricles on echocardiography and a single kidney on preliminary abdominal ultrasound. The infant also has difficulty feeding that results in coughing and apnea. Which of the following tissues was most likely affected during embryologic development?

A. Neuroectoderm
B. Mesoderm (Correct Answer)
C. Endoderm
D. Neural crest
E. Surface ectoderm

Explanation: ***Mesoderm*** - The presented constellation of anomalies is consistent with **VACTERL association** (Vertebral defects, Anal atresia, Cardiac defects, Tracheo-Esophageal fistula, Renal anomalies, and Limb abnormalities). - While VACTERL affects multiple germ layers, the **predominant tissue involved is mesoderm**, which gives rise to the cardiovascular system (ventricular septal defect), urogenital system (single kidney), skeletal system (limb and vertebral abnormalities), and connective tissue components of the GI tract (anal atresia musculature). - The **majority of the defects** in this case—cardiac (VSD), renal (single kidney), and limb (missing phalanges)—are primarily **mesodermal derivatives**, making mesoderm the most likely affected tissue. *Endoderm* - The endoderm forms the **epithelial lining of the gastrointestinal and respiratory tracts**, including the esophagus, trachea, and anal canal lining. - While the **tracheoesophageal fistula** and the epithelial component of **imperforate anus** do involve endodermal structures, the question asks for the **most likely** affected tissue, and the predominant pattern of defects points to mesoderm. - Endodermal involvement alone would not explain the cardiac, renal, and limb defects. *Neuroectoderm* - The neuroectoderm gives rise to the **central nervous system (brain and spinal cord)**, retina, posterior pituitary, and pineal gland. - While vertebral defects in VACTERL can be associated with neural tube defects, the primary anomalies described (cardiac, renal, limb, anal, TEF) are not primarily neuroectodermal in origin. *Neural crest* - Neural crest cells contribute to the **peripheral nervous system**, autonomic ganglia, melanocytes, craniofacial cartilage and bone, and parts of the heart (conotruncal septum and outflow tract). - While neural crest cells contribute to cardiac outflow tract development, a **ventricular septal defect** is primarily a mesodermal defect of the ventricular septum formation. - The constellation of renal, limb, and anal defects points away from primary neural crest involvement. *Surface ectoderm* - The surface ectoderm forms the **epidermis**, hair, nails, lens of the eye, tooth enamel, and anterior pituitary. - The described anomalies do not involve these surface structures and are not consistent with surface ectodermal defects.

Question 7: A 36-year-old man comes to the physician for a follow-up examination. Two weeks ago, he was diagnosed with an ischemic stroke of the right middle cerebral artery. He was treated with thrombolytics and does not have any residual symptoms. His pulse is 82/min and regular. Cardiovascular examination shows no abnormalities. Echocardiography shows a reproducible, transient, low-volume, right-to-left shunt through the atrial septum during coughing. Which of the following conditions is caused by failure of an embryologic process similar to that responsible for this patient's heart condition?

A. Transposition of the great vessels
B. Hirschsprung disease
C. Midgut volvulus
D. Thyroglossal duct cyst
E. Hypospadias (Correct Answer)

Explanation: ***Hypospadias*** - The patient's presentation with a **transient right-to-left shunt** through the atrial septum during coughing, especially after an ischemic stroke, suggests a **patent foramen ovale (PFO)**. A PFO results from the failed fusion of the **septum primum** and **septum secundum** postnatally. - Hypospadias is a developmental anomaly where the urethral opening is on the ventral side of the penis, resulting from the **incomplete fusion of the urethral folds** during embryogenesis. Both PFO and hypospadias arise from failures of fusion of embryonic structures. *Transposition of the great vessels* - This condition is caused by a failure of the **aorticopulmonary septum** to spiral during septation of the truncus arteriosus, leading to the aorta originating from the right ventricle and the pulmonary artery from the left ventricle. - This is a defect in **septation and spiraling**, not a failure of fusion of distinct embryonic structures. *Hirschsprung disease* - This is characterized by the **absence of ganglion cells** (Meissner and Auerbach plexuses) in the distal colon, due to the failure of **neural crest cell migration** to the affected bowel segments. - This condition is a defect in cell migration, distinct from a failure of embryonic structure fusion. *Midgut volvulus* - This occurs due to **malrotation of the intestines** during embryonic development, leading to an abnormally positioned small bowel that is prone to twisting around the superior mesenteric artery. - This is a defect in **intestinal rotation and fixation**, rather than a failure of fusion. *Thyroglossal duct cyst* - This results from the **failure of complete obliteration of the thyroglossal duct**, an embryonic remnant formed during the descent of the thyroid gland from the tongue base to its final position in the neck. - While it involves an embryonic remnant, it is a failure of **obliteration**, not fusion, of two separate structures.

Question 8: A newborn boy born vaginally to a healthy 37-year-old G3P1 from a pregnancy complicated by hydramnios fails to pass meconium after 24 hours of life. The vital signs are within normal limits for his age. The abdomen is distended, the anus is patent, and the rectal examination reveals pale mucous with non-pigmented meconium. Based on a barium enema, the boy is diagnosed with sigmoid colonic atresia. Disruption of which structure during fetal development could lead to this anomaly?

A. Inferior mesenteric artery (Correct Answer)
B. Superior mesenteric artery
C. Vitelline duct
D. Cloaca
E. Celiac artery

Explanation: ***Inferior mesenteric artery*** - **Sigmoid colonic atresia**, as observed in this case, results from an ischemic event affecting the segment of the bowel supplied by the **inferior mesenteric artery** during fetal development. - Interruption of blood flow to this region can lead to subsequent **atresia** as the affected part of the intestine necroses and is reabsorbed. *Superior mesenteric artery* - The **superior mesenteric artery** primarily supplies the midgut structures, including the small intestine and parts of the large intestine up to the transverse colon. - Disruption of the superior mesenteric artery would typically lead to atresias higher up in the **gastrointestinal tract**, such as jejunal or ileal atresias, not sigmoid colonic atresia. *Vitelline duct* - The **vitelline duct** (also known as the omphalomesenteric duct) connects the midgut to the yolk sac during early fetal development. - Persistent patency or partial obliteration of the vitelline duct can lead to anomalies like **Meckel's diverticulum** or vitelline cysts, which are distinct from colonic atresia. *Cloaca* - The **cloaca** is a common cavity for the digestive, urinary, and reproductive tracts during early embryonic development. - Defects in cloacal development lead to complex malformations involving these systems, such as **imperforate anus** or persistent cloaca, rather than isolated colonic atresia with a patent anus. *Celiac artery* - The **celiac artery** supplies the foregut structures, including the stomach, duodenum, liver, and spleen. - Disruption of the celiac artery during fetal development would result in malformations of these upper gastrointestinal organs, not the sigmoid colon.

Question 9: A research team is studying certain congenital anomalies of the respiratory tract. The method consists of marking a certain germinal layer with an isotope, following its development stages in chicken embryos, and finally analyzing the specimen. A given specimen of tissue is presented in the exhibit. Which of the following germinal structures most likely gave rise to the epithelial lining of this specimen?

A. Ectoderm
B. Neural crest
C. Mesoderm
D. Endoderm (Correct Answer)
E. Surface ectoderm

Explanation: ***Endoderm*** - The **epithelial lining** of the entire respiratory tract, including the larynx, trachea, bronchi, and lungs, is derived from the **endoderm**. - The **laryngotracheal groove** develops from the ventral wall of the primitive foregut, which is endodermal in origin, further differentiating into the respiratory tree. *Ectoderm* - The **ectoderm** primarily forms the epidermis, hair, nails, and the nervous system (brain and spinal cord). - It does not contribute to the internal epithelial lining of the respiratory tract. *Neural crest* - **Neural crest cells** contribute to a wide variety of structures, including components of the peripheral nervous system, head mesenchyme, and melanocytes. - They are not involved in forming the epithelial lining of the respiratory system. *Mesoderm* - The **mesoderm** forms the connective tissue, cartilage, and muscle components of the respiratory tract, such as the smooth muscle and cartilage rings of the trachea and bronchi, and the visceral pleura. - However, it does not form the epithelial lining itself. *Surface ectoderm* - **Surface ectoderm** specifically gives rise to the epidermis, hair, nails, and glands of the skin, as well as the oral cavity epithelium. - It does not contribute to the internal epithelial structures of the respiratory tract.

Question 10: A 26-year-old woman comes to the physician because she has not had a menstrual period for 5 weeks. Menarche was at the age of 14 years and menses occurred at regular 30-day intervals. She reports having unprotected sexual intercourse 3 weeks ago. A urine pregnancy test is positive. Which of the following best describes the stage of development of the embryo at this time?

A. Fetal heart is beating, but cardiac activity is not yet visible on ultrasound
B. Limb buds have formed, but fetal movements have not begun
C. Sexual differentiation has begun, but fetal movement has not started
D. Neural crest has formed, but limb buds have not yet formed (Correct Answer)
E. Implantation has occurred, but notochord has not yet formed

Explanation: ***Neural crest has formed, but limb buds have not yet formed*** - At **5 weeks gestational age (3 weeks post-fertilization)**, neurulation is completing or recently completed - **Neural crest cells** migrate from the neural folds during weeks 3-4 post-fertilization and are definitely present by this time - **Limb buds** appear later, around week 4-5 post-fertilization (week 6-7 gestational age), making this the most accurate description for the current developmental stage *Fetal heart is beating, but cardiac activity is not yet visible on ultrasound* - The primitive heart tube begins contracting around day 22-23 post-fertilization (early week 4) - At 3 weeks post-fertilization (5 weeks gestational age), the heart may just be starting to beat, but this timing is less precise - Cardiac activity becomes visible on transvaginal ultrasound around 5.5-6 weeks gestational age, so this option is close but less precise than the correct answer *Limb buds have formed, but fetal movements have not begun* - **Limb buds** typically appear around week 4-5 post-fertilization (week 6-7 gestational age) - This is **too advanced** for 3 weeks post-fertilization - While fetal movements aren't perceptible to the mother until 16-20 weeks, they begin much later than the current stage *Sexual differentiation has begun, but fetal movement has not started* - **Sexual differentiation** of the gonads begins around week 7 post-fertilization (week 9 gestational age) - External genitalia differentiation occurs even later (weeks 9-12 post-fertilization) - This stage is **far too advanced** for the current 3-week post-fertilization timeframe *Implantation has occurred, but notochord has not yet formed* - **Implantation** occurs 6-12 days after fertilization, which is approximately 2-3 weeks before a positive pregnancy test - The **notochord** forms during gastrulation in the **3rd week post-fertilization** (5th week gestational age) - By the time of this positive pregnancy test (5 weeks gestational age), the notochord has **already formed**, making this statement incorrect

Question 11: A 37-year-old man presents with dull, continuous epigastric pain that radiates to the back in a circumscribing fashion. The history is significant for 3 episodes of acute pancreatitis that were managed conservatively. He reports no history of such episodes in his relatives and denies a family history of any cardiovascular or gastrointestinal disorders. The vital signs include: blood pressure 105/70 mm Hg, heart rate 101/min, respiratory rate 17/min, and temperature 37.4℃ (99.3℉). The physical examination reveals epigastric tenderness, slight muscle guarding, a positive Mayo-Robson’s sign, and abdominal distention. Laboratory studies show the following findings: Complete blood count Erythrocytes 4.5 x 106/mm3 Hgb 14.7 g/dL Hct 43% Leukocytes 12,700/mm3 Segmented neutrophils 65% Bands 4% Eosinophils 1% Basophils 0% Lymphocytes 27% Monocytes 3% Biochemistry Serum amylase 170 U/L ALT 21 U/L AST 19 U/L Total serum cholesterol 139 mg/dL (3.6 mmol/L) Serum triglycerides 127 mg/dL (1.4 mmol/L) The magnetic resonance cholangiopancreatography findings are shown in the exhibit. What embryogenic disruption could cause such anatomic findings?

A. Duplication of the pancreatic bud of the midgut
B. Failure of fusion of dorsal and ventral pancreatic duct anlages (Correct Answer)
C. Ectopy of the developing bile duct
D. Duplication of the embryonic pancreatic duct
E. Improper rotation of the ventral pancreatic bud

Explanation: ***Failure of fusion of dorsal and ventral pancreatic duct anlages*** - The patient's history of **recurrent pancreatitis**, dull epigastric pain radiating to the back, and the provided magnetic resonance cholangiopancreatography (MRCP) findings are highly suggestive of **pancreas divisum**. Pancreas divisum results from the **failure of the dorsal and ventral pancreatic ducts to fuse** during embryological development. - In pancreas divisum, the majority of the pancreatic drainage occurs through the **minor papilla** via the dorsal pancreatic duct, which can become stenotic or obstructed, leading to increased pressure in the dorsal duct and recurrent pancreatitis. *Improper rotation of the anterior pancreatic bud* - **Improper rotation of the ventral pancreatic bud** (not anterior) can lead to **annular pancreas**, where pancreatic tissue encircles the duodenum. - While annular pancreas can cause duodenal obstruction, it does not typically present with the specific MRCP findings seen in pancreas divisum or recurrent pancreatitis as the primary symptom. *Duplication of the pancreatic bud of the midgut* - The pancreas develops from the **foregut**, not the midgut. Duplication of pancreatic buds is a rare condition and would likely manifest differently from the typical presentation of pancreas divisum. - Pancreatic development involves dorsal and ventral buds, not a "midgut" bud, making this option anatomically incorrect in the context of typical pancreatic anomalies. *Ectopy of the developing bile duct* - **Ectopy of the bile duct** refers to the bile duct opening in an abnormal location. While bile duct anomalies can occur, they are distinct from pancreatic duct fusion abnormalities. - This would primarily cause issues related to bile drainage, such as cholestasis or cholangitis, rather than the recurrent pancreatitis and specific ductal morphology associated with pancreas divisum. *Duplication of the embryonic pancreatic duct* - While congenital anomalies involving pancreatic ducts exist, **duplication of the entire embryonic pancreatic duct** is not a recognized cause of pancreas divisum. - Pancreas divisum is specifically due to a *failure of fusion* rather than an extra ductal structure stemming from an initial duplication.

Question 12: A 12-year-old girl with a recently diagnosed seizure disorder is brought to the physician by her mother for genetic counseling. She has difficulties in school due to a learning disability. Medications include carbamazepine. She is at the 95th percentile for height. Genetic analysis shows a 47, XXX karyotype. An error in which of the following stages of cell division is most likely responsible for this genetic abnormality?

A. Paternal meiosis, anaphase I
B. Maternal meiosis, telophase II
C. Maternal meiosis, metaphase II
D. Paternal meiosis, metaphase II
E. Maternal meiosis, anaphase II (Correct Answer)

Explanation: ***Maternal meiosis, anaphase II*** - A 47,XXX karyotype indicates an extra X chromosome, which most commonly results from **nondisjunction during maternal meiosis**. - **Nondisjunction in anaphase II** occurs when sister chromatids fail to separate properly, leading to an ovum containing two X chromosomes, which upon fertilization by an X-bearing sperm results in an XXX zygote. - While nondisjunction can occur in either meiosis I or II, both produce the same 47,XXX outcome; anaphase is the critical stage where chromosome separation occurs. *Paternal meiosis, anaphase I* - Nondisjunction during paternal meiosis I would lead to sperm with either no sex chromosomes or both X and Y chromosomes, resulting in **45,X0 (Turner syndrome) or 47,XXY (Klinefelter syndrome)** upon fertilization, not 47,XXX. - This involves improper segregation of **homologous chromosomes**. *Maternal meiosis, telophase II* - While telophase II is the final stage of meiosis II, the critical event of nondisjunction (failure of sister chromatids to separate) occurs during **anaphase II**, not telophase. - Errors in telophase II would affect nuclear reformation but do not cause the chromosomal separation failure responsible for **aneuploidy**. *Maternal meiosis, metaphase II* - Metaphase II is when sister chromatids align at the metaphase plate; while improper alignment could theoretically contribute to aneuploidy, the actual **physical separation failure** defining nondisjunction occurs during **anaphase II**. - Nondisjunction is specifically an anaphase phenomenon. *Paternal meiosis, metaphase II* - Paternal meiosis II nondisjunction would result in sperm carrying either two sex chromosomes (XX or YY) or none, leading to karyotypes like **47,XYY or 47,XXY**, not 47,XXX. - Paternal contribution of two X chromosomes would require the rare scenario of an X-bearing sperm with nondisjunction fertilizing an X-bearing ovum that also had nondisjunction, which is extremely unlikely.

Question 13: A 12-year-old boy follows up with his primary care physician for ongoing management of a urinary tract infection. He recently started middle school and has been having a difficult time navigating the school since he ambulates with leg braces and crutches. Consequently, he has not had sufficient time to use his urinary catheter appropriately. Otherwise, he has been unchanged from the previous visit with both sensory and motor defects in his lower extremities. He has had these defects since birth and has undergone surgeries to repair a bony defect in his spine with protrusion of a membrane through the defect. During what days of embryonic development did the defect responsible for this patient's symptoms most likely occur?

A. Days 21-35 (Correct Answer)
B. Days 0-7
C. Days 8-20
D. Days 90-birth
E. Days 36-90

Explanation: ***Days 21-35*** - This period during embryogenesis is crucial for **neurulation**, the process where the **neural tube** forms and closes. Failure of neural tube closure, particularly in the caudal region, leads to conditions like **spina bifida** (which aligns with the patient's described bony defect and neurological symptoms). - The patient's history of a **bony defect in the spine with protrusion of a membrane**, along with **sensory and motor defects in the lower extremities** and issues with bladder control (recurrent UTIs and catheter use), strongly indicates a **neural tube defect (NTD)**. Most NTDs occur between days 21 and 28 of embryonic development, encompassing the neural tube closure. *Days 0-7* - This initial period involves **fertilization, cleavage**, and **blastulation**. Cellular differentiation and major organ formation have not yet begun. - Defects during this stage typically result in early embryonic loss or very broad, systemic issues rather than specific structural defects like neural tube closure abnormalities. *Days 8-20* - This period includes implantation and early **gastrulation**, where the three germ layers (ectoderm, mesoderm, endoderm) are established. - While important developmental events occur, **neurulation**, the specific process affected in this patient, primarily begins around day 18-20 and continues into the next period. *Days 90-birth* - This period represents the **fetal stage**, characterized by growth and maturation of already formed organs and systems. - Major structural defects like spina bifida would have already occurred and been established much earlier in embryonic development. *Days 36-90* - This period, often referred to as the **fetal period of organogenesis**, involves significant growth and differentiation of organs. - By this stage, the neural tube would have already closed. While further development and refinement of the nervous system occur, the primary defect of **neural tube closure** would have happened before day 36.

Question 14: A 34-year-old woman comes to the emergency department because of a 2-hour history of abdominal pain, nausea, and vomiting that began an hour after she finished lunch. Examination shows abdominal guarding and rigidity; bowel sounds are reduced. Magnetic resonance cholangiopancreatography shows the dorsal pancreatic duct draining into the minor papilla and a separate smaller duct draining into the major papilla. The spleen is located anterior to the left kidney. A disruption of which of the following embryological processes is the most likely cause of this patient's imaging findings?

A. Proliferation of mesenchyme in the dorsal mesentery
B. Differentiation of the proximal hepatic diverticulum
C. Fusion of the pancreatic buds (Correct Answer)
D. Rotation of the midgut
E. Rotation of the dorsal mesogastrium

Explanation: ***Fusion of the pancreatic buds*** - The imaging findings describe **pancreas divisum**, a congenital anomaly where the **dorsal and ventral pancreatic buds fail to fuse** completely. - This leads to the dorsal pancreatic duct (containing most of the gland volume) draining through the **minor papilla**, which is functionally insufficient and can cause recurrent pancreatitis as seen in this patient with signs of peritonitis. *Proliferation of mesenchyme in the dorsal mesentery* - This process is primarily associated with the development of the **spleen**, which is derived from mesenchymal cells in the dorsal mesogastrium. - While the spleen's position is mentioned, its development or abnormal position is not the cause of the pancreatic ductal anomaly described. *Differentiation of the proximal hepatic diverticulum* - The **hepatic diverticulum** gives rise to the liver, gallbladder, and biliary tree. - Abnormalities in this process would lead to issues with these organs, not with the pancreatic ductal system. *Rotation of the dorsal mesogastrium* - The **dorsal mesogastrium** rotates during embryological development and gives rise to structures including the spleen and the greater omentum. - While abnormal rotation could explain splenic malposition, it does not directly explain the pancreatic ductal anomaly (pancreas divisum) described in the imaging. *Rotation of the midgut* - The **midgut rotation** is a complex embryological process involving the looping and rotation of the intestines around the superior mesenteric artery. - Errors in this process can lead to malrotation, volvulus, or intestinal atresia, but not pancreatic ductal anomalies.

Question 15: A 69-year-old smoker presents to physician after noticing that his face seems to be more swollen than usual. Upon further questioning, he reports increasing shortness of breath and cough over the past 6 months. On exam, his physician notices venous distention in his neck and distended veins in the upper chest and arms. Chest radiograph shows a right upper lobe mass. What is the embryologic origin of the vessel being compressed by this patient's tumor?

A. Cardinal veins (Correct Answer)
B. Primitive ventricle
C. Left horn of sinus venosus
D. Truncus arteriosus
E. Bulbus cordis

Explanation: ***Cardinal veins*** - The symptoms of facial swelling, neck vein distention, and upper chest/arm vein distention, especially with a right upper lobe mass, are classic for **superior vena cava (SVC) syndrome**. - The **SVC** is formed from the fusion of the anterior **cardinal veins**, which drain the upper body during embryonic development. *Primitive ventricle* - The **primitive ventricle** develops into parts of the left and right **ventricles** of the heart. - It is not directly involved in the formation of major systemic veins like the SVC. *Left horn of sinus venosus* - The **left horn of the sinus venosus** mostly regresses and contributes to structures like the **coronary sinus** and the oblique vein of the left atrium. - It does not form the SVC, which drains the upper body. *Truncus arteriosus* - The **truncus arteriosus** is an embryonic structure that separates to form the **aorta** and the **pulmonary artery**. - It is an arterial structure, not a venous structure that would be compressed in SVC syndrome. *Bulbus cordis* - The **bulbus cordis** develops into the **conus arteriosus** (infundibulum) of the right ventricle and the **aortic vestibule** of the left ventricle. - Like the truncus arteriosus, it is involved in arterial outflow tracts and not the formation of the SVC.

Question 16: A 21-year-old G3P2 woman presents to her obstetrician at 6 weeks gestation for routine prenatal care. Her past medical history includes obesity and gestational diabetes. She has had two spontaneous vaginal deliveries at term. One infant was macrosomic with hypoglycemia, but otherwise, she has had no complications. Her physician informs her that she must start taking a multivitamin with folic acid daily. The defect that folic acid supplementation protects against arises in tissue that is derived from which germ cell layer?

A. Mesoderm
B. Notochord
C. Endoderm
D. Mesenchyme
E. Ectoderm (Correct Answer)

Explanation: ***Ectoderm*** - Folic acid supplementation primarily prevents **neural tube defects**, such as **spina bifida** and **anencephaly**. - The **neural tube**, which forms the brain and spinal cord, is derived from the **ectoderm**. *Mesoderm* - The **mesoderm** gives rise to structures like muscle, bone, connective tissue, and the cardiovascular system. - Defects in mesodermal development are not primarily prevented by folic acid supplementation. *Notochord* - The **notochord** is a transient embryonic structure that induces the formation of the neural plate from the ectoderm. - While critical for nervous system development, it is not a germ cell layer itself, and defects in its development are not directly prevented by folic acid. *Endoderm* - The **endoderm** forms the lining of the gastrointestinal and respiratory tracts, as well as glands like the thyroid and pancreas. - Anomalies of these internal organs are not the primary target of folic acid supplementation. *Mesenchyme* - **Mesenchyme** is embryonic connective tissue, largely derived from the mesoderm, but can also come from neural crest (ectoderm). - It differentiates into connective tissues, blood, and lymphatic vessels; neural tube defects are not considered mesenchymal in origin.

Question 17: A child is in the nursery one day after birth. A nurse notices a urine-like discharge being expressed through the umbilical stump. What two structures in the embryo are connected by the structure that failed to obliterate during the embryologic development of this child?

A. Kidney - large bowel
B. Liver - umbilical vein
C. Bladder - small bowel
D. Pulmonary artery - aorta
E. Bladder - umbilicus (Correct Answer)

Explanation: ***Bladder - umbilicus*** - A **urine-like discharge** from the umbilical stump indicates a **patent urachus**, which is the embryonic remnant of the allantois. - The **allantois** (which becomes the urachus) is an embryonic structure that connects the **fetal bladder** to the **umbilicus** during development. - Normally, the allantois obliterates after birth to form the **median umbilical ligament**, but failure to obliterate results in a patent urachus allowing urine to discharge through the umbilicus. *Kidney - large bowel* - These two structures are not directly connected by an obliterating embryonic structure relevant to urine discharge from an umbilical stump. - The kidneys form urine, and the large bowel is part of the digestive tract, with no direct embryonic communication to the umbilicus for urine expression. *Liver - umbilical vein* - The umbilical vein connects the **placenta to the fetal liver** (and ductus venosus) to transport oxygenated blood, not urine. - Failure of the umbilical vein to obliterate would result in a patent umbilical vein, typically presenting as a vascular anomaly, not urine discharge. *Pulmonary artery - aorta* - These structures are connected by the **ductus arteriosus** in fetal circulation, bypassing the pulmonary circulation. - While important for fetal development, a patent ductus arteriosus (PDA) is a cardiovascular anomaly and would not manifest as urine discharge from the umbilical stump. *Bladder - small bowel* - While both structures are involved in waste elimination, there is no normal embryonic structure directly connecting the bladder and small bowel that obliterates to prevent urine discharge from the umbilicus. - An abnormal connection between the bladder and bowel would typically involve a **fistula** and present with stool in urine or urine in stool, not umbilical discharge.

Question 18: A 35-year-old man comes to the physician for evaluation of a neck mass and hoarseness. He has no history of major medical illness. Physical examination shows a 2.5-cm fixed, irregular thyroid nodule. His serum calcitonin concentration is elevated. The nodule is most likely comprised of cells that are embryologically derived from which of the following structures?

A. Second branchial pouch
B. Fourth branchial arch
C. Surface ectoderm
D. Third branchial pouch
E. Neural crest cells (Correct Answer)

Explanation: ***Neural crest cells*** - The elevated **serum calcitonin** in the presence of a thyroid nodule is highly suggestive of **medullary thyroid carcinoma (MTC)**. - **MTC** originates from the **parafollicular C cells** of the thyroid, which are embryologically derived from **neural crest cells** that migrate to the **ultimobranchial body** (from the 4th/5th pharyngeal pouch) during development and subsequently integrate into the thyroid gland. - This is the definitive embryological origin of calcitonin-producing C cells. *Second branchial pouch* - The second branchial pouch typically gives rise to the **palatine tonsil crypts** and **tonsillar fossa**. - It is not associated with the development of the **parafollicular C cells** or thyroid malignancies. *Fourth branchial arch* - The fourth branchial arch gives rise to the **superior laryngeal nerve** and associated laryngeal cartilages. - While the **ultimobranchial body** (from the 4th/5th pharyngeal pouch) does contribute C cells to the thyroid, these cells themselves are derived from **neural crest cells** that migrate to this structure, not from the pouch itself. - Therefore, the embryological origin is neural crest, not the branchial arch/pouch. *Surface ectoderm* - Surface ectoderm forms structures such as the **epidermis of the skin**, **hair**, **nails**, and **lens of the eye**. - It does not contribute to the development of the **C cells** or the thyroid gland. *Third branchial pouch* - The third branchial pouch gives rise to the **inferior parathyroid glands** and the majority of the **thymus**. - While it contributes to endocrine tissues, it does not form the **parafollicular C cells** of the thyroid.

Question 19: A new imaging modality is being tested to study vitelline duct morphology. A fetus at 20 weeks' gestation is found to have partial obliteration of this duct. Which of the following is the most likely sequela of this condition?

A. Discharge of urine from the umbilicus
B. Dilation of the descending colon
C. Swelling in the genital region
D. Protrusion of abdominal viscera into the umbilical cord
E. Bleeding from the gastrointestinal tract (Correct Answer)

Explanation: ***Bleeding from the gastrointestinal tract*** - Partial obliteration of the vitelline duct can lead to a **Meckel's diverticulum**, which is a remnant of the **yolk sac**. - Meckel's diverticulum may contain **ectopic gastric or pancreatic tissue**, leading to acid secretion, ulceration, and subsequent **gastrointestinal bleeding**. *Discharge of urine from the umbilicus* - This symptom is characteristic of a **patent urachus**, which is an abnormal connection between the **bladder and the umbilicus**, not related to the vitelline duct. - The urachus is a remnant of the **allantois**, a different embryonic structure. *Dilation of the descending colon* - **Dilation of the colon** is more commonly associated with conditions like **Hirschsprung's disease** or other causes of intestinal obstruction, which are not directly linked to vitelline duct anomalies. - Vitelline duct issues primarily affect the **small intestine** (ileum). *Swelling in the genital region* - Swelling in the genital region could indicate conditions such as a **hydrocele**, **hernia**, or **ambiguous genitalia**, which are unrelated to vitelline duct pathology. - Vitelline duct remnants typically present around the **umbilical region or within the small bowel**. *Protrusion of abdominal viscera into the umbilical cord* - This description is characteristic of an **omphalocele**, a congenital defect where abdominal contents herniate through the **umbilical ring**, covered by a sac. - While related to umbilical development, an omphalocele is a distinct condition from vitelline duct anomalies, which involve the vestigial connection between the **midgut and the yolk sac**.

Question 20: During the third week of development, the blastocyst undergoes a variety of differentiation processes responsible for the formation of the gastrula and, eventually, the embryo. This differentiation creates cell lineages that eventually become a variety of body systems. What cell lineage, present at this date, is responsible for the formation of the liver?

A. Neuroectoderm
B. Syncytiotrophoblasts
C. Ectoderm
D. Endoderm (Correct Answer)
E. Mesoderm

Explanation: ***Endoderm*** - The **endoderm** is one of the three primary germ layers that develops during gastrulation and is responsible for forming the lining of the **gastrointestinal tract** and associated organs, including the **liver** and pancreas. - Liver development begins from an outgrowth of the **foregut endoderm**, which differentiates into hepatocytes and bile duct cells, forming the hepatic parenchyma. *Neuroectoderm* - **Neuroectoderm** is a specialized part of the ectoderm that gives rise to the entire **nervous system**, including the brain, spinal cord, and peripheral nerves. - It does not contribute to the formation of visceral organs like the liver. *Syncytiotrophoblasts* - **Syncytiotrophoblasts** are a layer of the **trophoblast** that form part of the placenta, specifically involved in hormone production and nutrient exchange between the mother and fetus. - They are part of the supporting structures for pregnancy and do not contribute to the embryonic germ layers or organ formation within the embryo itself. *Ectoderm* - The **ectoderm** is the outermost germ layer and gives rise to the **epidermis of the skin**, hair, nails, nervous system, and sensory organs. - While it forms the outer coverings and nervous system, it does not directly form internal organs like the liver. *Mesoderm* - The **mesoderm** is the middle germ layer, responsible for forming **muscle**, **bone**, connective tissue, the circulatory system, kidneys, and gonads. - While mesoderm contributes supporting structures to the liver (blood vessels, connective tissue, hematopoietic cells), the **hepatic parenchyma** (hepatocytes and bile ducts) is derived from the endoderm, making endoderm the primary cell lineage responsible for liver formation.

Question 21: A mother brings her 3-day-old son to the pediatrician with a concern over drops of a clear yellow discharge from the clamped umbilical cord. These drops have formed every few hours every day. The vital signs are within normal limits and a cursory physical shows no abnormalities. On closer examination, the discharge is shown to be urine. The skin around the umbilical cord appears healthy and healing. The umbilical cord is appropriately discolored. An ultrasound shows a fistula tract that connects the urinary bladder and umbilicus. Which of the following structures failed to form in this patient?

A. Round ligament of the liver
B. Lateral umbilical ligament
C. Median umbilical ligament (Correct Answer)
D. Falciform ligament
E. Medial umbilical ligament

Explanation: ***Median umbilical ligament*** - The discharge of urine from the **umbilical cord stump** is indicative of a **patent urachus**, which occurs when the embryological connection between the bladder and umbilicus (the urachus) fails to close. - The urachus should normally obliterate and form the **median umbilical ligament** in adults, so its failure to form or close is the underlying cause for the fistula. *Round ligament of the liver* - This ligament is the remnant of the **umbilical vein** and is responsible for connecting the umbilicus to the liver. - A defect in the round ligament of the liver would typically present with symptoms related to hepatic circulation or umbilical hernias, not urinary discharge. *Lateral umbilical ligament* - This option likely refers to the **medial umbilical ligaments** (lateral umbilical folds), which are remnants of the **obliterated umbilical arteries**. - These paired structures carry blood from the internal iliac arteries to the placenta during fetal life and obliterate after birth. - Failure of these structures to obliterate would typically involve vascular issues, not a urinary fistula from the bladder. *Falciform ligament* - The falciform ligament is a **peritoneal fold** that attaches the liver to the anterior abdominal wall and contains the round ligament of the liver in its free edge. - While it is related to the umbilicus, its primary role is in supporting the liver, and its failure to form would not cause persistent urinary discharge from the umbilicus. *Medial umbilical ligament* - The medial umbilical ligaments are remnants of the **umbilical arteries**, which carry deoxygenated blood from the fetus to the placenta. - Failure of these structures to obliterate would typically involve vascular issues or persistent patent arteries, not a urinary fistula.

Question 22: Research is being conducted on embryoblasts. The exact date of fertilization is unknown. There is the presence of a cytotrophoblast and syncytiotrophoblast, marking the time when implantation into the uterus would normally occur. Within the embryoblast, columnar and cuboidal cells are separated by a membrane. Which of these cell layers begins to line the yolk sac cavity?

A. Hypoblast (Correct Answer)
B. Epiblast
C. Syncytiotrophoblast
D. Inner cell mass
E. Endoderm

Explanation: ***Hypoblast*** - The **hypoblast** is a layer of cuboidal cells that forms from the inner cell mass around day 8 post-fertilization. - It plays a crucial role in forming the **primary yolk sac** by migrating to line the exocoelomic cavity. *Epiblast* - The **epiblast** is composed of columnar cells located dorsal to the hypoblast and forms the floor of the **amniotic cavity**. - It is the source of the **three primary germ layers** during gastrulation (ectoderm, mesoderm, and endoderm), not the yolk sac lining itself. *Syncytiotrophoblast* - The **syncytiotrophoblast** is the outer, invasive layer of the trophoblast that facilitates implantation and forms the fetal component of the placenta. - It is not involved in lining the yolk sac cavity but rather in **invading the uterine endometrium** and producing hCG. *Inner cell mass* - The **inner cell mass (ICM)** is the cluster of cells within the blastocyst that gives rise to the embryoblast (which further differentiates into epiblast and hypoblast). - The ICM itself does not line the yolk sac; rather, its derivative, the hypoblast, does. *Endoderm* - The **endoderm** is one of the three primary germ layers that forms during gastrulation from the epiblast derivative. - It ultimately forms the linings of the **gastrointestinal and respiratory tracts**, not the primary yolk sac lining.

Question 23: A 31-year-old man presents for his annual physical exam. The physician fails to palpate a ductus deferens on the left side. An ultrasound confirms absence of the left ductus deferens and further reveals absence of the left epididymis, seminal vesicle, and kidney. Spermogram and reproductive hormones panel are within normal limits. Left-sided agenesis of an embryonic anlage is suspected. Which two structures are connected by this anlage during embryogenesis?

A. Metanephros and coelom
B. Mesonephros and coelom
C. Pronephros and cloaca
D. Mesonephros and cloaca (Correct Answer)
E. Pronephros and coelom

Explanation: ***Mesonephros and cloaca*** - The **mesonephros** forms the **mesonephric (Wolffian) duct**, which connects to the **cloaca**. - In males, this duct gives rise to the **epididymis**, **vas deferens**, **seminal vesicles**, and **ureteric bud** (which forms the kidney collecting system). The simultaneous absence of the kidney, epididymis, and seminal vesicle indicates a defect in this developmental pathway. *Metanephros and coelom* - The **metanephros** is a later stage of kidney development, forming the definitive kidney parenchyma, but it does **not connect directly to the coelom**. - Its excretory ducts develop from an interaction with the ureteric bud (derived from the mesonephric duct), not directly from the coelom. *Mesonephros and coelom* - While the **mesonephros** develops within the intermediate mesoderm adjacent to the **coelom**, its primary connection relevant to these structures is to the cloaca via the mesonephric duct. - The **coelom** is the primitive body cavity, and direct connections to the adult reproductive/urinary tracts in this manner are not embryologically accurate for this defect. *Pronephros and cloaca* - The **pronephros** is a transient, non-functional embryonic kidney structure that degenerates early in development and does **not form the definitive male reproductive or urinary structures** seen here. - Its connection to the cloaca is not as direct or as developmentally critical for the structures described in the case. *Pronephros and coelom* - Similar to the previous option, the **pronephros** is an early and transient structure. - It does **not develop into the adult genitourinary structures** affected in this patient, and its relationship with the coelom is not the key to understanding this specific congenital anomaly.

Question 24: A 13-year-old girl presents to an orthopedic surgeon for evaluation of a spinal curvature that was discovered during a school screening. She has otherwise been healthy and does not take any medications. On presentation, she is found to have significant asymmetry of her back and is sent for a spine radiograph. The radiograph reveals a unilateral rib attached to the left transverse process of the C7 vertebrae. Abnormal expression of which of the following genes is most likely responsible for this finding?

A. WNT7
B. FGF
C. Homeobox (Correct Answer)
D. PAX
E. Sonic hedgehog

Explanation: ***Homeobox*** - **Homeobox genes (HOX genes)** play a crucial role in specifying the identity of vertebral segments along the **craniocaudal axis** during embryonic development. - An abnormal expression of these genes can lead to **skeletal malformations**, such as the formation of a **cervical rib**, by altering the segmental identity of the C7 vertebra. *WNT7* - **WNT7 genes** are involved in limb patterning and have a role in the formation of the **dorsoventral axis** of the limb and kidney development. - They are not primarily associated with vertebral segmentation or the formation of cervical ribs. *FGF* - **Fibroblast growth factor (FGF) genes** are essential for various processes, including limb development, neurogenesis, and angiogenesis. - While they are involved in numerous developmental pathways, they are not the primary genes responsible for specifying vertebral identity and thus cervical rib formation. *PAX* - **PAX genes** are a family of transcription factors critical for organ development, especially of the eye, brain, and kidney. - While important for development, they are not directly implicated in the specification of vertebral segments or the pathogenesis of cervical ribs. *Sonic hedgehog* - **Sonic hedgehog (SHH)** signaling is a key pathway in embryonic development, particularly for pattern formation in the neural tube, limbs, and facial structures. - While crucial for body axis development and segmentation, **HOX genes** have a more direct role in determining the specific identity of vertebral segments and causing cervical ribs.

Question 25: A 22-year-old G2P1 female presents to the clinic at the beginning of her third trimester for a fetal ultrasound. The sonographer is unable to visualize any of the structures arising from the mesonephric duct in the female fetus. This infant is at risk for malformation of which of the following?

A. Fallopian tubes
B. Upper 1/3 of vagina
C. Uterus
D. No malformation would be expected (Correct Answer)
E. Lower 2/3 of vagina

Explanation: ***No malformation would be expected*** - In females, structures originating from the **mesonephric duct** (Wolffian duct) normally **regress** due to the absence of testosterone. - The female reproductive tract develops from the **paramesonephric duct** (Müllerian duct), which differentiates into the fallopian tubes, uterus, and upper vagina. *Fallopian tubes* - The **fallopian tubes** develop from the **paramesonephric (Müllerian) duct**, not the mesonephric duct. - Therefore, non-visualization of mesonephric duct structures would not affect their development. *Upper 1/3 of vagina* - The **upper 1/3 of the vagina** also develops from the **paramesonephric (Müllerian) duct**. - Its formation is independent of the mesonephric duct. *Uterus* - The **uterus** originates from the fusion of the two **paramesonephric (Müllerian) ducts**. - Its development is not dependent on mesonephric duct structures. *Lower 2/3 of vagina* - The **lower 2/3 of the vagina** develops from the **urogenital sinus**, which is distinct from both the mesonephric and paramesonephric ducts. - Thus, it would not be affected by non-development or non-visualization of mesonephric duct structures.

Question 26: A 2-year-old boy is brought to the emergency department by his mother for evaluation of severe abdominal pain that began one hour ago. On examination, the patient is afebrile and has diffuse rebound tenderness with acute epigastric pain. A stool guaiac test is positive. A small bowel perforation is suspected. What is the embryologic structure that is the underlying cause of this patient’s presentation?

A. Fibrous cord remnant
B. Anal membrane
C. Vermiform appendix
D. Cloaca
E. Vitelline duct (Correct Answer)

Explanation: ***Vitelline duct*** - The symptoms of **severe abdominal pain**, rebound tenderness, epigastric pain, and **positive stool guaiac** (indicating bleeding) in a 2-year-old suggest a bleeding **Meckel's diverticulum**, which is a remnant of the **vitelline duct**. - **Meckel's diverticulum** is the most common congenital anomaly of the GI tract, often containing **ectopic gastric or pancreatic tissue** that can lead to ulceration, bleeding, or perforation. *Fibrous cord remnant* - While a fibrous cord remnant can be associated with the **vitelline duct**, it typically presents with **intestinal obstruction (volvulus or intussusception)** rather than perforation and bleeding from ectopic mucosa. - A fibrous cord is a potential complication of a persistent **vitelline duct**, but it is not the underlying embryologic structure responsible for ectopic tissue or bleeding. *Anal membrane* - The **anal membrane** is involved in the development of the **anus and rectum**. - Persistence of the **anal membrane** would lead to **anal atresia** or stenosis, causing symptoms of difficult defecation or obstruction, not abdominal pain and perforation like in this case. *Vermiform appendix* - The **vermiform appendix** is a lymphoid organ arising from the **cecum**. - While **appendicitis** can cause severe abdominal pain and perforation, a positive stool guaiac and presentation with ectopic gastric tissue leading to ulceration are not characteristic features. *Cloaca* - The **cloaca** is a common embryologic structure that divides into the **urogenital sinus** and the **anorectal canal**. - Abnormalities of the **cloaca** typically result in complex **anomalies of the urogenital and GI tracts**, such as persistent cloaca with a single perineal opening, not an isolated perforation with bleeding.

Question 27: A 52-year-old woman sees you in your office with a complaint of new-onset headaches over the past few weeks. On exam, you find a 2 x 2 cm dark, irregularly shaped, pigmented lesion on her back. She is concerned because her father recently passed away from skin cancer. What tissue type most directly gives rise to the lesion this patient is experiencing?

A. Neural crest cells (Correct Answer)
B. Endoderm
C. Mesoderm
D. Ectoderm
E. Neuroectoderm

Explanation: ***Neural crest cells*** - The suspected lesion, given its description and the patient's family history of skin cancer, is likely a **melanoma**. - Melanoma originates from **melanocytes**, which are derived from **neural crest cells** during embryonic development. *Endoderm* - The endoderm gives rise to the **lining of the gastrointestinal and respiratory tracts**, as well as organs such as the liver and pancreas. - It is not involved in the formation of melanocytes or skin lesions like melanoma. *Mesoderm* - The mesoderm forms tissues such as **muscle, bone, cartilage, connective tissue**, and the circulatory system. - It does not directly give rise to melanocytes, which are the cells of origin for melanoma. *Ectoderm* - The ectoderm gives rise to the **epidermis, nervous system**, and sensory organs. - While melanocytes are found in the epidermis, they are specifically derived from the **neural crest (a sub-population of ectoderm)**, not the general ectoderm. *Neuroectoderm* - Neuroectoderm specifically refers to the ectoderm that develops into the **nervous system**. - While neural crest cells originate from the neuroectoderm, "neural crest cells" is a more precise answer for the origin of melanocytes.

Question 28: A 10-year-old boy is brought to his pediatrician over concern for a 2-month history of headaches. Recently, the patient has been experiencing nausea and vomiting, along with some difficulty with coordination during soccer practice last week. On exam, the patient's temperature is 98.2°F (36.8°C), blood pressure is 110/80 mmHg, pulse is 72/min, and respirations are 14/min. On further evaluation, the patient is found to have a well-encapsulated posterior fossa mass. The patient undergoes surgical resection, and the mass is found to be positive for GFAP. Which of the following is derived from the same embryologic germ layer as the cells that comprise this tumor?

A. Ependymal cells (Correct Answer)
B. Nucleus pulposus
C. Schwann cells
D. Microglia
E. Melanocytes

Explanation: ***Ependymal cells*** - The tumor described is a **pilocytic astrocytoma**, a common **posterior fossa tumor** in children, characterized by its **well-encapsulated** nature and **GFAP positivity**, indicating an **astrocytic origin**. - **Astrocytes** develop from the **neuroectoderm/neural tube**, as do **ependymal cells**, which line the ventricles and central canal of the spinal cord. *Nucleus pulposus* - The **nucleus pulposus** is a remnant of the **notochord**, which originates from the **mesoderm**. - This embryological origin is distinct from the **neuroectodermal** origin of the tumor cells. *Schwann cells* - **Schwann cells** are derived from the **neural crest**, which is also **ectodermal** but represents a different lineage than the **neural tube**. - While both contribute to the nervous system, **neural crest cells** form the peripheral nervous system, whereas the **neural tube** forms the central nervous system. *Microglia* - **Microglia** are the immune cells of the CNS and are derived from the **mesoderm**, specifically from **monocytes** in the yolk sac. - Their origin is distinct from the **neuroectodermal** lineage of astrocytes. *Melanocytes* - **Melanocytes** are pigment-producing cells found in the skin, hair, and eyes, and they are derived from the **neural crest**. - This makes their embryological origin different from the **neural tube** cells that give rise to astrocytes.

Question 29: A 29-year-old mother brings in her 2-week-old baby boy to a pediatrician because he has been having difficulty feeding. The mother reveals that she had no prenatal care during her pregnancy and gave birth at home without complications. She says that her son seems to be having difficulty sucking, and she occasionally sees breast milk coming out of the infant’s nose. Physical exam reveals that this patient has a gap between his oral and nasal cavities behind the incisive foramen. He is therefore prescribed specialized bottles and his mom is taught positional techniques to ensure better feeding. Failure to fuse which of the following structures is most likely responsible for this patient's disorder?

A. Maxillary and medial nasal prominences
B. Nasal septum with primary plates
C. Maxillary and lateral nasal prominences
D. Palatine shelves with primary plates
E. Palatine shelves with nasal septum (Correct Answer)

Explanation: ***Palatine shelves with nasal septum*** - A **cleft palate** results from the **failure of fusion of the palatine shelves** with each other and/or with the **nasal septum**, creating an abnormal communication between the oral and nasal cavities. - This anatomical defect explains the infant's **feeding difficulties** and the leakage of breast milk into the nose, as well as the observed **gap behind the incisive foramen**. *Maxillary and medial nasal prominences* - The failure of fusion between the maxillary and medial nasal prominences results in a **cleft lip**, which is an anterior defect and does not explain the posterior gap described. - While cleft lip can coexist with cleft palate, the symptoms here specifically point to a palatal defect, not primarily a lip defect. *Nasal septum with primary plates* - The primary palate forms from the fusion of the medial nasal prominences, anterior to the incisive foramen. - While crucial for normal development, the specific clinical presentation (gap *behind* the incisive foramen and feeding difficulties) is more characteristic of a secondary palate defect involving the palatine shelves. *Maxillary and lateral nasal prominences* - The fusion of these structures contributes to the formation of the **nasolacrimal groove** and parts of the cheek, not the palate. - Deficiencies in this fusion would lead to defects in the lateral facial region, not an oro-nasal communication related to feeding. *Palatine shelves with primary plates* - The **primary palate** fuses with the anterior part of the secondary palate (formed by the palatine shelves) at the incisive foramen. - However, the more common and clinically relevant defect leading to an open communication between the oral and nasal cavities, especially *behind* the incisive foramen, involves the failure of fusion of the **palatine shelves** with each other and the **nasal septum**.

Question 30: A six year-old female presents for evaluation of dry skin, fatigue, sensitivity to cold and constipation. The patient’s mother recalls that the patient had surgery to remove a “benign mass” at the base of her tongue 3 months ago because of trouble swallowing. What was the likely cause of the surgically removed mass?

A. Failed caudal migration of the thyroid gland (Correct Answer)
B. Iodine deficiency
C. Failed fusion of the palatine shelves with the nasal septum
D. Radiation exposure
E. Maternal Diabetes Mellitus

Explanation: ***Failed caudal migration of the thyroid gland*** - The symptoms of **dry skin, fatigue, cold sensitivity, and constipation** are classic signs of **hypothyroidism**. - A mass at the **base of the tongue** that is later removed and leads to hypothyroidism strongly suggests a **lingual thyroid**, which results from the thyroid gland's **failure to descend** from its embryonic origin at the foramen cecum. *Iodine deficiency* - While iodine deficiency can cause **hypothyroidism** and **goiter**, it typically does not present as a **discrete mass** at the base of the tongue requiring surgical removal. - It would usually result in a more generalized **enlargement of the thyroid gland**, often in the neck. *Failed fusion of the palatine shelves with the nasal septum* - This developmental anomaly leads to a **cleft palate**, affecting the **roof of the mouth**. - It has no direct association with thyroid function or masses at the base of the tongue. *Radiation exposure* - Radiation exposure can increase the risk of thyroid cancer or hypothyroidism later in life, but it typically doesn't cause a congenital benign mass at the base of the tongue that presents in early childhood. - Furthermore, the specific presentation points to a developmental anomaly rather than an acquired condition. *Maternal Diabetes Mellitus* - Maternal diabetes can lead to various congenital anomalies in the fetus, such as **macrosomia, caudal regression syndrome, or cardiac defects**. - However, it is not a recognized cause of a lingual thyroid or a failure of thyroid migration.

Question 31: A male newborn is born at 37 weeks' gestation after spontaneous vaginal delivery. The mother had no prenatal care. Physical examination shows a urethral opening on the dorsal aspect of the penis, 4 mm proximal to the glans. There is a 3-cm defect in the midline abdominal wall superior to the pubic symphysis with exposure of moist, erythematous mucosa. Which of the following is the most likely underlying cause of this patient's findings?

A. Abnormal development of the gubernaculum
B. Persistence of the urogenital membrane
C. Failed fusion of the urethral folds
D. Incomplete union of the labioscrotal swellings
E. Malpositioning of the genital tubercle (Correct Answer)

Explanation: ***Malpositioning of the genital tubercle*** - This condition describes **epispadias** (urethral opening on the dorsal aspect) and **bladder exstrophy** (abdominal wall defect with exposed bladder mucosa), which are frequently co-occurring and caused by a **ventral displacement of the genital tubercle**. - A defect in the **abdominal wall closure** allows the bladder to remain exposed, and the abnormal positioning of the genital tubercle leads to an improperly formed urethra. *Abnormal development of the gubernaculum* - The gubernaculum is crucial for **testicular descent**; abnormal development would typically lead to **cryptorchidism** (undescended testes). - It does not explain the urethral opening on the dorsal penis or the abdominal wall defect. *Persistence of the urogenital membrane* - Persistent urogenital membrane typically results in conditions like an **imperforate hymen** in females or **urogenital sinus anomalies**, but not epispadias or bladder exstrophy. - This membrane normally ruptures, creating the definitive openings for the urethra and vagina. *Failed fusion of the urethral folds* - Failed fusion of the urethral folds in males can lead to **hypospadias**, where the urethral opening is on the **ventral** aspect of the penis. - It does not account for the **dorsal urethral opening (epispadias)** described or the associated bladder exstrophy. *Incomplete union of the labioscrotal swellings* - Incomplete union of the labioscrotal swellings in males results in **bifid scrotum** or **hypospadias**. - This doesn't explain the characteristic dorsal urethral defect of epispadias or the large abdominal wall defect associated with bladder exstrophy.

Question 32: A 32-year-old G1P0 woman undergoes her 2nd-trimester ultrasound in a community hospital. During her prenatal care, she was found to have mild anemia, low levels of folate, and serum alpha-fetoprotein levels greater than 2 multiples of the median (MoM) on 2 separate occasions. Her 1st-trimester ultrasound was significant for the absence of the intracranial lucency, no visualization of the cisterna magna, and posterior shift of the brain stem. These 2nd-trimester ultrasound reports reveal the widening of the lumbosacral spine ossification centers and the presence of a sac in proximity to the lumbosacral defect. Which of the following statements best describes the congenital defect in the fetus?

A. Abnormal development of the caudal eminence
B. Persistence of the anterior accessory neurenteric canal (ANC)
C. Failure of the caudal neuropore to close (Correct Answer)
D. Failure of mesenchymal cells to form a neural rod
E. Failure of the rostral neuropore to close

Explanation: ***Failure of the caudal neuropore to close*** - The constellation of findings, including **elevated alpha-fetoprotein (AFP)**, absence of intracranial lucency, no visualization of the cisterna magna, posterior shift of the brain stem, widening lumbosacral spine ossification centers, and a sac near a lumbosacral defect, points to a **neural tube defect**, specifically an **open spina bifida** (myelomeningocele). - This condition results from the **incomplete closure of the neural tube**, particularly the **caudal neuropore**, which normally closes around day 27-28 of embryonic development. *Abnormal development of the caudal eminence* - **Caudal eminence defects** typically manifest as caudal regression syndrome, which involves abnormalities of the sacrum and lower limbs, but usually **not** an open neural tube defect with elevated AFP and characteristic cranial ultrasound findings. - While there is a lumbosacral defect, the extensive features point away from an isolated caudal eminence issue. *Persistence of the anterior accessory neurenteric canal (ANC)* - **Persistent ANC** is a rare condition that can lead to **duplications of the neural tube** or gastrointestinal tract, often associated with a connection between the gut and the neural canal. - This would not explain the high AFP, absence of intracranial lucency, or the specific sonographic features of an open neural tube defect like spina bifida. *Failure of mesenchymal cells to form a neural rod* - The formation of the neural tube is a complex process; while mesenchymal cells are involved in surrounding structures, the neural rod itself primarily forms from the **neuroectoderm**. - This description does not accurately reflect the embryological origin of spina bifida, which is fundamentally a defect of neural tube closure rather than a failure of neural rod formation. *Failure of the rostral neuropore to close* - Failure of the **rostral (cranial) neuropore** to close leads to **anencephaly** or encephalocele, characterized by the absence of a significant portion of the brain and skull. - While AFP levels would be high, the ultrasound findings of a sac in the lumbosacral region and the specific intracranial findings (e.g., absence of intracranial lucency, posterior shift of the brain stem, which are signs of Arnold-Chiari malformation type II) are much more consistent with a caudal defect like myelomeningocele.

Question 33: A 25-year-old man presents to the clinic with a midline swelling in his neck. He is unsure about when it appeared. He denies any difficulty with swallowing or hoarseness. His past medical history is insignificant. On physical examination, there is a 1 cm x 2 cm firm mildly tender nodule on the anterior midline aspect of the neck which moves with deglutition and elevates with protrusion of the tongue. Which of the following is the most likely embryologic origin of the nodule in this patient?

A. Midline endoderm of the pharynx (Correct Answer)
B. 1st and 2nd pharyngeal arch
C. The branchial cleft
D. 4th pharyngeal arch
E. 4th pharyngeal pouch

Explanation: ***Midline endoderm of the pharynx*** - The symptoms described, particularly a midline neck swelling that **moves with deglutition** and **elevates with tongue protrusion**, are classic for a **thyroglossal duct cyst**. - Thyroglossal duct cysts arise from remnants of the **thyroglossal duct**, an embryonic structure that forms from the **midline endoderm of the pharyngeal floor** and descends to form the thyroid gland. *1st and 2nd pharyngeal arch* - The 1st and 2nd pharyngeal arches primarily contribute to the formation of structures in the **mandible**, **maxilla**, **middle ear**, and **hyoid bone**. - Abnormalities in these arches typically lead to conditions like **Treacher Collins syndrome** or **Pierre Robin sequence**, not midline neck cysts with these specific movement characteristics. *The branchial cleft* - **Branchial cleft cysts** typically present as **lateral neck masses**, often anterior to the sternocleidomastoid muscle, and usually do not move with deglutition or tongue protrusion. - They arise from incomplete obliteration of **pharyngeal clefts**, which are ectodermal structures. *4th pharyngeal arch* - The 4th pharyngeal arch contributes to the formation of the **cricothyroid muscle**, part of the **pharynx**, and the **laryngeal cartilages**. - Anomalies of the 4th pharyngeal arch are rare and typically involve **vascular structures** or **recurrent laryngeal nerve** abnormalities, not midline neck cysts. *4th pharyngeal pouch* - The 4th pharyngeal pouch contributes to the development of the **superior parathyroid glands** and the **ultimobranchial body** (which gives rise to parafollicular C cells of the thyroid). - Malformations of this pouch are associated with parathyroid and thyroid conditions, not midline thyroglossal duct cysts.

Question 34: After a year of trying to conceive, a young couple in their early twenties decided to try in vitro fertilization. During preliminary testing of fertility, it was found that the male partner had dysfunctional sperm. Past medical history revealed that he had frequent sinus and lung infections throughout his life. The physician noted an abnormal exam finding on palpation of the right fifth intercostal space at the midclavicular line. What would be the most likely diagnosis responsible for this patient's infertility?

A. Chédiak-Higashi syndrome
B. Kartagener syndrome (Correct Answer)
C. Adenosine deaminase deficiency
D. Williams syndrome
E. Cystic fibrosis

Explanation: ***Kartagener syndrome*** - This syndrome is a subgroup of **primary ciliary dyskinesia** characterized by the triad of **situs inversus**, **chronic sinusitis**, and **bronchiectasis**. The abnormal exam finding on palpation of the right fifth intercostal space at the midclavicular line suggests the apex beat is on the right side, indicating **dextrocardia** (situs inversus) which supports this diagnosis. - The **dysfunctional sperm** and **frequent respiratory infections** are also classic features, resulting from immotile cilia in the respiratory tract and immotile flagella in sperm. *Chédiak-Higashi syndrome* - This is a rare autosomal recessive disorder characterized by **immunodeficiency**, **partial oculocutaneous albinism**, and **peripheral neuropathy**. - While patients suffer from recurrent infections due to impaired phagolysosome formation, it does not typically cause situs inversus or male infertility due to dysfunctional sperm. *Adenosine deaminase deficiency* - This is a severe form of **severe combined immunodeficiency (SCID)**, leading to a profound deficiency of T and B lymphocytes. - Patients typically present with **recurrent opportunistic infections** and failure to thrive early in life, but it does not cause situs inversus or issues with sperm motility. *Williams syndrome* - This is a genetic condition characterized by specific **facial features** (e.g., "elfin" facies), **cardiovascular problems** (especially supravalvular aortic stenosis), unique **cognitive profiles**, and **overly friendly personalities**. - It does not involve recurrent respiratory infections, situs inversus, or male infertility. *Cystic fibrosis* - This genetic disorder primarily affects the **lungs**, **pancreas**, liver, and intestines, causing the production of thick, sticky mucus. - While it causes **recurrent lung infections** and **male infertility** (due to congenital bilateral absence of the vas deferens, not immotile sperm), it does not cause situs inversus.

Question 35: A newborn girl is delivered vaginally at term to a healthy 25-year-old G1P1. The pregnancy was uncomplicated. On examination, she was found to have a slight anal invagination, but no opening. Further examination shows a vestibular fistula and normally developed external genitalia. Which of the following statements about this condition is correct?

A. Such abnormal anatomy is formed after week 12 of intrauterine development.
B. The presence of an associated perineal or vestibular fistula is more likely in females with trisomy 21.
C. There is a failure of the division of the embryonic cloaca into the urogenital sinus and rectoanal canal.
D. Other congenital abnormalities are extremely rare in patients with this condition.
E. There is a failure of the rupture of the anal membrane (dorsal portion of the cloacal membrane). (Correct Answer)

Explanation: ***There is a failure of the rupture of the anal membrane (dorsal portion of the cloacal membrane).*** - This clinical presentation describes an **imperforate anus** with a **vestibular fistula**, a type of anorectal malformation (ARM). - During normal development, the **urorectal septum** divides the cloaca into the urogenital sinus (ventral) and anorectal canal (dorsal), and the **cloacal membrane** is similarly divided into the **urogenital membrane** and **anal membrane**. - The **anal membrane** (the dorsal portion of the cloacal membrane) normally ruptures around week 8 to form the anal opening. Failure of this rupture results in **imperforate anus**. - The presence of a **vestibular fistula** indicates an abnormal communication between the rectal pouch and the vestibule, reflecting incomplete separation of the urogenital and anorectal structures. *Such abnormal anatomy is formed after week 12 of intrauterine development.* - **Anorectal malformations (ARMs)**, including imperforate anus with fistula, arise from defects in the development of the **cloaca** and **urorectal septum**, which occurs between **weeks 4 and 7** of gestation. - By week 12, organogenesis is largely complete, and such gross anatomical defects would have already occurred earlier in embryonic development. *The presence of an associated perineal or vestibular fistula is more likely in females with trisomy 21.* - While anorectal malformations can be associated with various chromosomal anomalies, **Trisomy 21 (Down syndrome)** is not specifically associated with a higher incidence of vestibular fistulas. - Anorectal malformations are more commonly associated with **VACTERL association** (Vertebral defects, Anal atresia, Cardiac defects, Tracheoesophageal fistula, Esophageal atresia, Renal anomalies, and Limb defects), which typically occurs sporadically without a specific chromosomal abnormality. *There is a failure of the division of the embryonic cloaca into the urogenital sinus and rectoanal canal.* - While incomplete cloacal septation contributes to anorectal malformations, a **complete failure** of cloacal division typically results in a **persistent cloaca**, where there is a single common opening for the urinary, genital, and gastrointestinal tracts. - The described presentation has normally developed external genitalia with a separate vestibular fistula, indicating that cloacal division occurred but was abnormal, and the primary defect is failure of anal membrane rupture. *Other congenital abnormalities are extremely rare in patients with this condition.* - **Anorectal malformations (ARMs)** are frequently associated with other congenital anomalies, with approximately **50-60%** of affected individuals having at least one additional defect. - Associated anomalies commonly include components of **VACTERL association** and involve the genitourinary system (30-50%), vertebral/skeletal system (30%), cardiac system (10-20%), and gastrointestinal tract (esophageal atresia, duodenal atresia).

Question 36: A mother brings her 1-week-old son to the pediatrician because she is concerned about the child’s umbilicus. She notes that there appears to be fluid draining from the child’s umbilicus several times a day. The child has been breastfeeding normally. On exam, a small amount of clear light yellow fluid drains from the child’s umbilical stump when pressure is applied to the child’s lower abdomen. No bilious or feculent drainage is noted. Which of the following embryologic structures is associated with this patient’s condition?

A. Paramesonephric duct
B. Urachus (Correct Answer)
C. Umbilical vein
D. Ductus venosus
E. Omphalomesenteric duct

Explanation: ***Urachus*** - The draining of clear, light yellow fluid from the umbilical stump on abdominal pressure is characteristic of a **patent urachus**, which is a remnant of the **allantois**. - A patent urachus connects the **umbilical cord** to the **bladder**, allowing urine to leak through the umbilicus. *Paramesonephric duct* - The paramesonephric (Müllerian) duct develops into female reproductive structures like the **fallopian tubes**, **uterus**, and **upper vagina**. - It is not associated with umbilical drainage or the urinary system. *Umbilical vein* - The umbilical vein carries **oxygenated blood** from the placenta to the fetus and typically obliterates to form the **ligamentum teres hepatis**. - Its patency would lead to vascular issues, not fluid drainage from the umbilicus. *Ductus venosus* - The ductus venosus shunts **oxygenated blood** from the umbilical vein directly to the inferior vena cava, bypassing the liver. - It obliterates to form the **ligamentum venosum** and is not involved in umbilical fluid leakage. *Omphalomesenteric duct* - The omphalomesenteric (vitelline) duct connects the **fetal midgut** to the **yolk sac** during early development. - A patent omphalomesenteric duct would typically present with **feculent** or **bilious drainage** from the umbilicus, not clear, light yellow fluid, as it is connected to the digestive tract.

Question 37: A 2-year-old male is brought to his pediatrician by his mother because of abdominal pain and blood in the stool. Scintigraphy reveals uptake in the right lower quadrant of the abdomen. Persistence of which of the following structures is the most likely cause of this patient's symptoms?

A. Ureteric bud
B. Urachus
C. Paramesonephric duct
D. Omphalomesenteric duct (Correct Answer)
E. Allantois

Explanation: ***Omphalomesenteric duct*** - Persistence of the **omphalomesenteric duct** (vitelline duct) can lead to a **Meckel's diverticulum**, which often contains **ectopic gastric or pancreatic tissue**. - This ectopic tissue can cause **ulceration and bleeding**, leading to abdominal pain and **melena** (blood in stool), and will show uptake on a Technetium-99m pertechnetate scan (scintigraphy) for ectopic gastric mucosa. *Ureteric bud* - The ureteric bud forms the **ureter, renal pelvis, calyces**, and **collecting ducts** of the kidney. - Anomalies of the ureteric bud typically present with **urinary tract issues**, such as hydronephrosis or renal agenesis, not abdominal pain and bloody stools from GI bleeding. *Urachus* - The urachus is a remnant of the **allantois** connecting the fetal bladder to the umbilicus. - Persistent urachal remnants can cause urine leakage from the umbilicus, cysts, or infections, but generally not abdominal pain and bloody stools. *Paramesonephric duct* - The paramesonephric (Müllerian) ducts form the **female reproductive organs** (fallopian tubes, uterus, cervix, upper vagina). - Persistence or anomalies of these ducts are relevant to **gynecological issues** and infertility in females, not the GI symptoms described in a male child. *Allantois* - The allantois contributes to the formation of the **urachus** and is involved in early blood formation and bladder development. - While related to the urachus, its direct persistence or anomalies typically don't cause the specific presentation of abdominal pain and bloody stools due to ectopic gastric mucosa.

Question 38: An investigator is studying the teratogenicity of cigarette smoking during pregnancy. He reviews several databases containing data about birth defects and prenatal drug exposures and finds that infants exposed to cigarette smoke in utero are approximately 2 times as likely to have a particular birth defect than unexposed infants. This defect results from abnormal development during the 6th week of gestation, when the maxillary prominences grow medially and fuse first with the lateral and then the medial nasal prominence. The defect is most likely which of the following?

A. Cleft palate
B. Macrognathia
C. Torus palatinus
D. Choanal atresia
E. Cleft lip (Correct Answer)

Explanation: ***Cleft lip*** - **Cleft lip** results from the incomplete fusion of the **medial nasal prominence** and the **maxillary prominence**, which normally occurs around the 6th week of gestation. - Exposure to **cigarette smoking** during pregnancy is a known **teratogen** that increases the risk of this developmental abnormality. *Cleft palate* - **Cleft palate** involves the incomplete fusion of the **palatal shelves**, occurring later in gestation (weeks 7-12), and is a separate developmental defect from cleft lip. - While smoking can increase the risk of cleft palate, the described embryological event (fusion of maxillary and nasal prominences) specifically leads to **cleft lip**. *Macrognathia* - **Macrognathia** refers to an abnormally large jaw, which is a growth anomaly rather than a failure of fusion of facial prominences. - This condition is not directly related to the developmental processes described in the question. *Torus palatinus* - **Torus palatinus** is a benign bony protuberance on the hard palate, often a genetically inherited trait, and is not a birth defect caused by failed embryonic fusion. - It develops much later in life or can be present from birth but does not involve the specific embryonic structures mentioned. *Choanal atresia* - **Choanal atresia** is a blockage of the posterior nasal passage, caused by the failure of the **nasal cavity** to communicate with the **nasopharynx**. - This defect is not related to the fusion of the maxillary and nasal prominences, which forms the upper lip and primary palate.

Question 39: A male newborn is delivered at term to a 26-year-old woman, gravida 2, para 3. The mother has no medical insurance and did not receive prenatal care. Physical examination shows microcephaly and ocular hypotelorism. There is a single nostril, cleft lip, and a solitary central maxillary incisor. An MRI of the head shows a single large ventricle and fused thalami. This patient's condition is most likely caused by abnormal expression of which of the following protein families?

A. Wnt
B. Hedgehog (Correct Answer)
C. Homeobox
D. Fibroblast growth factor
E. Transforming growth factor

Explanation: ***Hedgehog*** - The presented clinical features—**microcephaly**, **ocular hypotelorism**, **single nostril**, **cleft lip**, **solitary central maxillary incisor**, and neuroimaging findings of a **single large ventricle and fused thalami**—are classic manifestations of **holoprosencephaly**. - **Holoprosencephaly** is a severe developmental anomaly caused by the incomplete division of the prosencephalon (forebrain) and is strongly associated with mutations in genes involved in the **Hedgehog signaling pathway**, particularly the **Sonic Hedgehog (SHH)** gene. *Wnt* - The **Wnt signaling pathway** is crucial for various developmental processes, including **neural tube closure**, limb patterning, and organogenesis. - Abnormalities in Wnt signaling are associated with conditions like **neural tube defects** and specific cancers, but not typically with the facial and brain malformations seen in holoprosencephaly. *Homeobox* - **Homeobox (Hox) genes** are a family of transcription factors that play a critical role in patterning the body axis during embryonic development, determining the identity of body segments. - Mutations in **Hox genes** are linked to various congenital anomalies, especially affecting the **skeletal system** and **limbs**, but do not directly cause the classic features of holoprosencephaly. *Fibroblast growth factor* - **Fibroblast growth factors (FGFs)** and their receptors are involved in a wide range of developmental processes, including **limb development**, **bone formation**, and **neurogenesis**. - Dysregulation of FGF signaling is associated with conditions like **craniosynostosis** and various skeletal dysplasias, but not the specific brain and facial abnormalities observed in holoprosencephaly. *Transforming growth factor* - The **Transforming Growth Factor-beta (TGF-β) superfamily** includes a diverse group of growth factors involved in cell growth, differentiation, apoptosis, and extracellular matrix production. - Dysfunction in TGF-β signaling is implicated in conditions like **Marfan syndrome** and various fibrotic disorders, but it is not the primary pathway linked to the pathogenesis of holoprosencephaly.

Question 40: A 26-year-old gravida-1-para-0 (G-1-P-0) presents for a routine prenatal check-up at 16 weeks gestation. The patient has no concerns but is excited to learn the gender of the baby. Genetic testing was performed that showed an XY genotype; however, an ultrasound does not reveal the development of external male genitalia. Which of the following is responsible for the initial step of the development of male characteristics?

A. Formation of the paramesonephric duct
B. Formation of the genital ridge
C. Production of anti-Mullerian hormone
D. SRY gene product (Correct Answer)
E. Conversion of testosterone to DHT

Explanation: ***SRY gene product*** - The **SRY gene**, located on the **Y chromosome**, is the **master switch** for male sexual differentiation, encoding the **testis-determining factor (TDF)**. - TDF induces the bipotential gonads to differentiate into **testes**, which subsequently produce hormones essential for male development. *Formation of the paramesonephric duct* - The **paramesonephric (Müllerian) duct** is critical for the development of **female internal reproductive organs** (fallopian tubes, uterus, upper vagina). - Its formation does not initiate male characteristics; rather, its **regression** is a hallmark of male development due to **anti-Müllerian hormone (AMH)**. *Formation of the genital ridge* - The **genital ridge** is the embryonic precursor for both male and female gonads (**bipotential gonad**). - Its formation is a **sex-indifferent** stage and precedes the genetic signaling that directs differentiation towards either testes or ovaries. *Production of anti-Mullerian hormone* - **Anti-Müllerian hormone (AMH)** is produced by the **Sertoli cells** of the developing testes. - Its primary function is to cause the **regression of the paramesonephric (Müllerian) ducts**, preventing the formation of female internal genitalia, which is a *later step* in male differentiation following testis formation. *Conversion of testosterone to DHT* - The conversion of **testosterone to dihydrotestosterone (DHT)** by **5-alpha-reductase** is essential for the development of **external male genitalia** (e.g., penis, scrotum). - This process occurs *after* the testes have formed and begun producing testosterone, and it is responsible for the *final virilization* rather than the initial step of male characteristics.

Question 41: A 7-year-old boy presents with difficulty swallowing, diarrhea, itching, and weakness. He also complains of episodes of headaches, sweating, and palpitations, which are accompanied by fear and tend to end with micturition. His mother is concerned about the strange nodules on his lips and eyelids. The boy's younger brother had similar nodules and died at 10 years of age of unknown causes. The patient’s vital signs are as follows: blood pressure 130/80 mm Hg, heart rate 107/min, respiratory rate 14/min, and temperature 36.9℃ (98.4℉). The child is tall, thin, has disproportionately long arms and legs, and increased thoracic spine kyphosis. There are multiple yellow-white, sessile, painless nodules on the patient’s lips, and buccal and eyelid mucosa. There is a painless lump in the area of the left thyroid lobe and enlargement of the posterior cervical lymph nodes on the left side. What is the most probable embryonic origin of the cells in the lump?

A. First pharyngeal groove
B. Neural crest cells (Correct Answer)
C. First pharyngeal pouch
D. Lateral plate mesoderm
E. Neurogenic placodes

Explanation: ***Neural crest cells*** - This constellation of symptoms, including diarrhea, hypertension, sweating, and palpitations, along with a thyroid lump and genetic predisposition (brother's death), are highly suggestive of **Multiple Endocrine Neoplasia type 2B (MEN2B)**. - **Medullary thyroid carcinoma (MTC)**, a characteristic component of MEN2B, originates from the **parafollicular C cells** of the thyroid gland, which are derived from **neural crest cells**. *First pharyngeal groove* - The **first pharyngeal groove** primarily contributes to the formation of structures like the **external auditory meatus**. - It is not associated with the development of the thyroid C cells or the pathology described. *First pharyngeal pouch* - The **first pharyngeal pouch** gives rise to structures such as the **auditory tube (Eustachian tube)** and the **tympanic cavity**. - It is not involved in the embryological origin of thyroid C cells. *Lateral plate mesoderm* - The **lateral plate mesoderm** forms components like the **parietal and visceral layers of serous membranes**, cardiovascular system, and some connective tissues. - It does not contribute to the development of the parafollicular C cells of the thyroid. *Neurogenic placodes* - **Neurogenic placodes** are ectodermal thickenings that give rise to various sensory structures, such as the **nasal placode** (giving rise to olfactory epithelium) and the **otic placode** (giving rise to the inner ear). - They are distinct from the neural crest cells and not involved in the origin of parafollicular C cells.

Question 42: A 17-year old girl is brought to the physician by her mother because menarche has not yet occurred. She is at the 3rd percentile for height. Examination of a buccal mucosal scraping shows no Barr bodies. Karyotyping of a neutrophil shows 45 chromosomes. Which of the following is the most likely underlying cause of this patient's cytogenetic abnormality?

A. Postzygotic mitotic error
B. Uniparental isodisomy
C. Robertsonian translocation
D. Reciprocal translocation
E. Nondisjunction during meiosis I (Correct Answer)

Explanation: ***Nondisjunction during meiosis I*** - The **45 chromosome karyotype** with **no Barr bodies** indicates **Turner syndrome (45,X)**, where the patient has only one X chromosome - Barr body count follows the formula n-1 (where n = number of X chromosomes); with 1 X chromosome, there are **0 Barr bodies**, which matches the clinical finding - **Nondisjunction during meiosis I** is the most common mechanism causing aneuploidy, where homologous chromosomes fail to separate properly during the first meiotic division, resulting in gametes with missing or extra chromosomes - This patient's presentation of **primary amenorrhea** and **short stature** (3rd percentile) are classic features of Turner syndrome - Nondisjunction can occur in either parent; maternal nondisjunction increases with advanced maternal age *Postzygotic mitotic error* - A postzygotic mitotic error would result in **mosaicism** (e.g., 45,X/46,XX), where some cells have a normal karyotype and others are abnormal - The description indicates a consistent finding of **45 chromosomes** in the neutrophil, suggesting **non-mosaic Turner syndrome** rather than a postzygotic error affecting only a subset of cells - Mosaic Turner syndrome typically presents with milder features *Uniparental isodisomy* - Uniparental isodisomy involves inheritance of two identical copies of a chromosome (or chromosomal region) from one parent - This mechanism maintains a **normal chromosome number of 46**, just with altered parental origin - Does not explain the **45 chromosome** count observed in this patient - Associated with imprinting disorders when affecting imprinted regions *Robertsonian translocation* - Robertsonian translocation involves fusion of two acrocentric chromosomes (13, 14, 15, 21, 22) at their centromeres with loss of short arms - While this can reduce the chromosome count to 45, the individual typically has a **balanced translocation** with preservation of essential genetic material - Does not result in monosomy X or Turner syndrome phenotype - More commonly associated with Down syndrome in offspring of balanced translocation carriers *Reciprocal translocation* - Reciprocal translocation involves exchange of segments between two non-homologous chromosomes - Balanced carriers have **46 chromosomes** with rearranged genetic material - While unbalanced segregation can produce aneuploid gametes, this is not the direct mechanism for **monosomy X** - Does not explain the specific loss of an entire sex chromosome

Question 43: A 29-year-old woman comes to the physician because of poor balance and recurrent falls for the past month. She has also had blurry vision in her right eye for the past 2 weeks. She reports worsening of her symptoms after taking warm baths. Physical examination shows generalized hyperreflexia and an intention tremor. Romberg sign is positive. Visual acuity is 20/50 in the left eye and 20/100 in the right eye, and she is unable to distinguish red from green colors. The cells primarily affected by this patient's condition are most likely derived from which of the following embryologic structures?

A. Mesoderm
B. Neural crest
C. Endoderm
D. Notochord
E. Neuroectoderm (Correct Answer)

Explanation: ***Neuroectoderm*** - This patient presents with symptoms highly suggestive of **multiple sclerosis (MS)**, including **neurological deficits** disseminated in time and space, **optic neuritis** (blurry vision, color desaturation), and **worsening of symptoms with heat** (Uhthoff's phenomenon). - MS is characterized by the **demyelination of oligodendrocytes** in the central nervous system (CNS); **oligodendrocytes** and other glial cells of the CNS (astrocytes, ependymal cells) are derived from the **neuroectoderm**. *Mesoderm* - The mesoderm gives rise to **muscle**, **bone**, **connective tissue**, **kidneys**, and the **circulatory system**, but not the oligodendrocytes affected in this condition. - Though some components of the nervous system (e.g., microglia, which are macrophages) are mesodermal in origin, the primary pathogenesis of MS involves **neuroectoderm-derived oligodendrocytes**. *Neural crest* - Neural crest cells form components of the **peripheral nervous system (PNS)**, such as **Schwann cells**, **ganglia**, and **adrenal medulla**, as well as other diverse tissues like melanocytes and craniofacial bones. - MS is a disease of the **central nervous system**, and the cells primarily affected (oligodendrocytes) are not derived from the neural crest. *Endoderm* - The endoderm forms the **epithelial lining of the gastrointestinal** and **respiratory tracts**, and associated glands like the liver and pancreas. - It does not contribute to the development of the neurons or glial cells of the central nervous system. *Notochord* - The notochord is a transient embryonic structure that induces the formation of the **neural plate** from the ectoderm and later becomes the **nucleus pulposus of the intervertebral discs**. - While it plays a crucial role in nervous system development, it does not directly differentiate into the cells primarily affected in MS.

Question 44: A mother brings her 2-year-old son to the pediatrician following an episode of abdominal pain and bloody stool. The child has otherwise been healthy and growing normally. On physical exam, the patient is irritable with guarding of the right lower quadrant of the abdomen. Based on clinical suspicion, pertechnetate scintigraphy demonstrates increased uptake in the right lower abdomen. Which of the following embryologic structures is associated with this patient’s condition?

A. Paramesonephric duct
B. Metanephric mesenchyme
C. Vitelline duct (Correct Answer)
D. Ductus arteriosus
E. Allantois

Explanation: ***Vitelline duct*** - This clinical presentation (a 2-year-old with abdominal pain, bloody stool, and increased pertechnetate uptake in the right lower abdomen) is highly suggestive of a **Meckel's diverticulum**. - A **Meckel's diverticulum** is a remnant of the **vitelline duct** (also known as the omphalomesenteric duct), a structure that connects the yolk sac to the midgut in the embryo. *Paramesonephric duct* - The **paramesonephric ducts (Müllerian ducts)** develop into female reproductive organs (fallopian tubes, uterus, cervix, and upper vagina). - Remnants of this duct are not associated with gastrointestinal bleeding or masses in the right lower abdomen in males. *Metanephric mesenchyme* - The **metanephric mesenchyme** forms the nephrons and collecting ducts of the kidney, thus giving rise to the kidneys. - Abnormalities are related to kidney development (e.g., renal agenesis, dysplastic kidney), not gastrointestinal issues like those described. *Ductus arteriosus* - The **ductus arteriosus** is a fetal blood vessel connecting the pulmonary artery to the aorta, which typically closes shortly after birth. - Failure of closure results in a **patent ductus arteriosus**, causing cardiovascular symptoms (e.g., heart murmur, differential cyanosis), not gastrointestinal bleeding. *Allantois* - The **allantois** is an embryonic structure involved in waste removal and gas exchange; its remnant in adults is the **urachus**. - Persistent urachal remnants can cause umbilical discharge or recurrent urinary tract infections, which are not consistent with the patient's symptoms.

Question 45: A 2-day-old boy fails to pass meconium for the first 48 hours of life. He was born at term to a healthy 19-year-old woman after an uncomplicated pregnancy. At birth, his weight was 3.9 kg (8.6 lb); at the time of presentation, he weighs 3.8 kg (8.4 lb). His vital signs are as follows: blood pressure 70/50 mm Hg, heart rate 130/min, respiratory rate 33/min, and temperature 37.0℃ (98.6℉). On physical examination, he is fussy and appears mildly dehydrated. Bowel sounds are active on auscultation. His abdomen is mildly distended and no masses can be identified on palpation. The patient’s anus is patent. An upper gastrointestinal study with oral contrast demonstrates normal anatomy. A lower gastrointestinal series with barium enema reveals a large amount of retained barium contrast within a dilated sigmoid colon and a normal appearing rectum. The barium solution retention persisted beyond 24 hours after administration. Which of the following best describes the cause of the patient’s symptoms?

A. Decreased blood supply to developing intestine in the embryonic period
B. Hypertrophy of the muscular layer of the lower sigmoid colon
C. Propelling of a polyp distally by peristalsis
D. Disruption of apoptosis of intestinal cells
E. Failure of neural crest cells to migrate caudally to intestinal wall during embryogenesis (Correct Answer)

Explanation: ***Failure of neural crest cells to migrate caudally to intestinal wall during embryogenesis*** - The presentation of a newborn with **failure to pass meconium**, abdominal distention, and a dilated sigmoid colon with a normal rectum on barium enema is highly indicative of **Hirschsprung disease**. - This condition results from the **failure of neural crest cells** to migrate completely to the distal bowel during embryogenesis, leading to the **absence of ganglion cells** in the affected segment (usually rectum and distal colon). *Decreased blood supply to developing intestine in the embryonic period* - Decreased blood supply during embryonic development is typically associated with **intestinal atresia** or **stenosis**, which would present with different imaging findings (e.g., complete obstruction, "double bubble" sign). - This cause does not explain the characteristic dilated proximal colon and aganglionic distal segment seen in Hirschsprung disease. *Hypertrophy of the muscular layer of the lower sigmoid colon* - While the muscular layers can hypertrophy proximal to an obstruction, **pyloric stenosis** is a more common condition involving muscular hypertrophy causing obstruction, and it affects the stomach outlet, not the colon. - The underlying issue in this case is the **absence of nerve cells**, not primary muscular hypertrophy leading to obstruction in the colon. *Propelling of a polyp distally by peristalsis* - While polyps can cause obstruction, they are exceedingly **rare in newborns** as a cause of meconium retention or intestinal obstruction. - This mechanism would not explain the imaging findings of a dilated sigmoid colon and a normal rectum with retained barium. *Disruption of apoptosis of intestinal cells* - Disruption of apoptosis can lead to various developmental abnormalities but is not the known mechanism for **Hirschsprung disease**. - The primary issue is a **defect in neural crest cell migration** resulting in aganglionosis.

Question 46: A newborn male born prematurely at 33 weeks is noted to have mild dyspnea and difficulty with feeding. Examination reveals bounding peripheral radial pulses and a continuous 'machine-like' murmur. The patient is subsequently started on indomethacin. Which of the following is the embryologic origin of the structure most likely responsible for this patient's presentation?

A. 4th branchial arch
B. 4th branchial pouch
C. 6th aortic arch (Correct Answer)
D. 1st branchial cleft
E. 3rd aortic arch

Explanation: ***6th aortic arch*** - The patient's presentation with **dyspnea**, feeding difficulties, **bounding peripheral pulses**, and a **continuous "machine-like" murmur** in a premature infant is highly suggestive of a **patent ductus arteriosus (PDA)**. - The **ductus arteriosus** is derived from the **6th aortic arch** (specifically the distal left 6th arch). After birth, it normally closes to become the **ligamentum arteriosum**. - **Indomethacin** is used to pharmacologically close a PDA by inhibiting prostaglandin synthesis, which maintains ductal patency. *4th branchial arch* - The **4th branchial arch** contributes to the formation of the **aortic arch** on the left and the **subclavian artery** on the right, but not the ductus arteriosus itself. - Problems with the 4th branchial arch can lead to conditions like a **double aortic arch** or an **aberrant subclavian artery**. *4th branchial pouch* - The **branchial pouches** are endodermal structures involved in the development of various organs like the **thymus** and **parathyroid glands**. - The 4th branchial pouch specifically gives rise to the **superior parathyroid glands**. - They do not contribute to the formation of the great vessels or the ductus arteriosus. *1st branchial cleft* - The **branchial clefts** (or pharyngeal clefts) are ectodermal structures that primarily contribute to the formation of the **external auditory meatus** and cervical sinuses/cysts. - They are not involved in the development of cardiovascular structures like the ductus arteriosus. *3rd aortic arch* - The **3rd aortic arch** contributes to the formation of the **common carotid arteries** and the proximal part of the **internal carotid arteries**. - It does not give rise to the ductus arteriosus.

Question 47: A 35-year-old female presents to her gynecologist complaining of vaginal discomfort. She reports that over the past two weeks, she has developed dyspareunia and feels as if there is a mass on the external aspect of her vagina. She is sexually active in a monogamous relationship. On physical examination, there is a 2 cm unilateral erythematous swelling on the right side of the posterolateral labia minora. Which of the following embryologic precursors gives rise to the affected tissue in this patient?

A. Urogenital fold
B. Paramesonephric duct
C. Labioscrotal swelling
D. Genital tubercle
E. Urogenital sinus (Correct Answer)

Explanation: ***Urogenital sinus*** - This patient presents with symptoms and signs consistent with a **Bartholin's gland cyst** or abscess due to its location on the posterolateral labia minora. - The Bartholin's glands, also known as the **greater vestibular glands**, develop from endodermal buds that arise from the **urogenital sinus** epithelium. - The urogenital sinus gives rise to the **bladder**, **urethra**, **lower one-third of the vagina**, **vestibule of the vagina**, and the **Bartholin's glands and Skene's glands** (paraurethral glands). *Paramesonephric duct* - The paramesonephric (Müllerian) ducts develop into the **fallopian tubes**, **uterus**, and the **upper two-thirds of the vagina**. - They do not contribute to the formation of the external genitalia or Bartholin's glands. *Labioscrotal swelling* - In females, the labioscrotal swellings develop into the **labia majora**. - In males, these swellings fuse to form the **scrotum**. *Urogenital fold* - In females, the urogenital folds remain largely unfused and form the **labia minora**. - In males, these folds fuse to enclose the penile urethra and form the **ventral aspect of the penis**. - They do not give rise to glandular structures. *Genital tubercle* - The genital tubercle is an embryonic structure that develops into the **clitoris** in females. - In males, it develops into the **glans penis**.

Question 48: A 4-week-old boy is brought to the emergency department with a 2-day history of projectile vomiting after feeding. His parents state that he is their firstborn child and that he was born healthy. He developed normally for several weeks but started to eat less 1 week ago. Physical exam reveals a small, round mass in the right upper quadrant of the abdomen close to the midline. The infant throws up in the emergency department, and the vomitus is observed to be watery with no traces of bile. Which of the following is associated with the most likely cause of this patient's symptoms?

A. Chloride transport defect
B. Recanalization defect
C. Vascular accident
D. Failure of neural crest migration
E. Nitric oxide synthase deficiency (Correct Answer)

Explanation: ***Nitric oxide synthase deficiency*** - This symptom constellation points to **pyloric stenosis**, which is characterized by smooth muscle hypertrophy and concurrent deficiency of **nitric oxide synthase** and possibly **interstitial cells of Cajal** in the pylorus. - The thickened pylorus creates an obstruction, leading to non-bilious projectile vomiting, an "olive-like" mass, and subsequent electrolyte imbalances. *Chloride transport defect* - A chloride transport defect, particularly in the **CFTR (cystic fibrosis transmembrane conductance regulator) gene**, is characteristic of cystic fibrosis. - While cystic fibrosis can cause gastrointestinal symptoms like **meconium ileus** or malabsorption, it does not typically present with projectile vomiting and a palpable abdominal mass in this age group, nor is it the primary cause of pyloric stenosis. *Recanalization defect* - **Recanalization defects** refer to issues in the development of a lumen in structures that are initially solid during embryogenesis, such as the gut tube. - This can lead to conditions like **duodenal atresia**, which typically presents with **bilious vomiting** (due to obstruction distal to the ampulla of Vater) and often a "double-bubble" sign on imaging, which is not described here. *Vascular accident* - A **vascular accident** in the gut can lead to conditions such as **necrotizing enterocolitis** in neonates or **bowel ischemia/infarction**. - These conditions would typically present with symptoms like bloody stools, abdominal distension, lethargy, and systemic signs of illness, rather than isolated projectile vomiting and an abdominal mass. *Failure of neural crest migration* - **Failure of neural crest cell migration** is the underlying cause of **Hirschsprung disease**, where there is an absence of ganglion cells in the distal colon. - This typically presents with **constipation**, abdominal distension, and failure to pass meconium, rather than projectile vomiting, and usually affects the large intestine.

Question 49: An investigator is studying the effects of an antihypertensive drug during pregnancy. Follow-up studies show that the drug can adversely affect differentiation of the ureteric bud into its direct derivatives in fetuses exposed during the first trimester. Which of the following structures is most likely to develop incorrectly in the affected fetus?

A. Collecting ducts (Correct Answer)
B. Proximal convoluted tubule
C. Loop of Henle
D. Bladder
E. Distal convoluted tubule

Explanation: ***Collecting ducts*** - The **ureteric bud** is an outgrowth of the **mesonephric (Wolffian) duct** that directly gives rise to the **collecting ducts**, minor and major calyces, renal pelvis, and ureter. - An adverse effect on the differentiation of the ureteric bud during the first trimester would directly impact the development of these structures, potentially leading to renal dysplasia or collecting system abnormalities. - **Clinical correlation**: ACE inhibitors and ARBs are contraindicated in pregnancy due to their teratogenic effects on fetal renal development. *Proximal convoluted tubule* - The **proximal convoluted tubule** develops from the **metanephric mesenchyme**, not from the ureteric bud. - This structure is part of the nephron proper, which forms when the metanephric mesenchyme is induced by the ureteric bud to differentiate. *Loop of Henle* - The **loop of Henle** also develops from the **metanephric mesenchyme**. - While its formation depends on inductive signals from the ureteric bud, it is not a direct derivative of the ureteric bud itself. *Bladder* - The **bladder** develops from the **urogenital sinus**, which is derived from the ventral part of the **cloaca** after partitioning by the urorectal septum. - Its development is distinct from the derivatives of the ureteric bud, though they are functionally connected. *Distal convoluted tubule* - The **distal convoluted tubule** develops from the **metanephric mesenchyme**. - It is a component of the nephron and connects to the collecting duct but is not a direct derivative of the ureteric bud.

Question 50: A 3175-g (7-lb) male newborn is delivered at 39 weeks' gestation to a 29-year-old primigravid woman following a spontaneous vaginal delivery. Apgar scores are 8 and 9 at 1 and 5 minutes, respectively. Cardiac examination in the delivery room shows a continuous machine-like murmur. An echocardiogram shows a structure with blood flow between the pulmonary artery and the aorta. This structure is most likely a derivate of which of the following?

A. 4th aortic arch
B. 1st aortic arch
C. 6th aortic arch (Correct Answer)
D. 2nd aortic arch
E. 3rd aortic arch

Explanation: ***6th aortic arch*** - The description of a "continuous machine-like murmur" and a structure with blood flow between the pulmonary artery and the aorta is characteristic of a **patent ductus arteriosus (PDA)**. - The **ductus arteriosus** is a remnant of the **6th aortic arch**, connecting the pulmonary artery to the aorta in fetal life. *4th aortic arch* - The **4th aortic arch** contributes to the formation of the **aortic arch** itself on the left side and the proximal **right subclavian artery** on the right. - Abnormalities of the 4th arch can lead to conditions like **coarctation of the aorta** or **vascular rings**, which do not typically present as a PDA. *1st aortic arch* - The **1st aortic arch** largely disappears, but its remnants contribute to the formation of the **maxillary artery** and the **external carotid artery**. - It is not involved in developmental anomalies of the major vessels between the pulmonary artery and aorta. *2nd aortic arch* - The **2nd aortic arch** also largely regresses, but its remnants contribute to the **stapedial artery** and part of the **hyoid artery**. - It does not play a role in the formation of the ductus arteriosus or other major arteries of the heart. *3rd aortic arch* - The **3rd aortic arch** develops into the common carotid arteries and the proximal internal carotid arteries. - Genetic disorders and malformations involving this arch typically affect the carotid system, not the connection between the pulmonary artery and aorta.

Question 51: A researcher is studying the brains of patients who recently died from stroke-related causes. One specimen has a large thrombus in an area of the brain that is important in relaying many modalities of sensory information from the periphery to the sensory cortex. Which of the following embryologic structures gave rise to the part of the brain in question?

A. Metencephalon
B. Diencephalon (Correct Answer)
C. Mesencephalon
D. Telencephalon
E. Myelencephalon

Explanation: ***Diencephalon*** - The **thalamus**, a key relay center for sensory information to the cerebral cortex, develops from the diencephalon. - A thrombus in this area would severely impair the transmission of **sensory modalities** from the periphery. *Metencephalon* - The metencephalon gives rise to the **pons** and the **cerebellum**. - These structures are primarily involved in motor control, coordination, and respiratory regulation, not direct sensory relay to the cortex. *Mesencephalon* - The mesencephalon develops into the **midbrain**. - The midbrain contains structures involved in visual and auditory reflexes, and motor control, but not the primary sensory relay described. *Myelencephalon* - The myelencephalon gives rise to the **medulla oblongata**. - The medulla is crucial for vital autonomic functions (e.g., breathing, heart rate) and connects the brain to the spinal cord. *Telencephalon* - The telencephalon develops into the **cerebral hemispheres** (cerebral cortex, basal ganglia, hippocampus). - While it processes sensory information, it is not the primary relay center from the periphery; that role belongs to the thalamus.

Question 52: An 11-year-old girl is brought to the office by her mother due to complaint of intermittent and severe periumbilical pain for 1 day. She does not have any significant past medical history. She provides a history of a recent school trip to the suburbs. On physical examination, there is a mild tenderness around the umbilicus without any distension or discharge. There is no rebound tenderness. Bowel sounds are normal. An abdominal imaging shows enlarged mesenteric lymph nodes, and she is diagnosed with mesenteric lymphadenitis. However, incidentally, a mass of tissue was seen joining the inferior pole of both kidneys as shown in the image. Which of the following best describes this renal anomaly?

A. Fused kidneys ascend beyond superior mesenteric artery.
B. Rapid progression to acute renal failure
C. Kidneys are usually non-functional.
D. Increased risk of developing renal vein thrombosis
E. Association with ureteropelvic junction obstruction (UPJO) (Correct Answer)

Explanation: ***Association with ureteropelvic junction obstruction (UPJO)*** - **Horseshoe kidney** is characterized by the fusion of the lower poles (most common) or upper poles of the kidneys, forming a U-shape. This anomaly is associated with an increased incidence of **ureteropelvic junction obstruction (UPJO)** due to the abnormal course of the ureters over the isthmus. - The abnormal ascent of the fused kidneys can also lead to an increased incidence of other anomalies such as **vesicoureteral reflux**, **renal calculi**, and recurrent urinary tract infections. *Fused kidneys ascend beyond superior mesenteric artery.* - The **horseshoe kidney** typically **fails to ascend** completely during development because its isthmus (the fused part) can get trapped under the **inferior mesenteric artery**. - Therefore, fused kidneys in horseshoe kidney are often found in a **lower position** than normal, not ascended beyond the superior mesenteric artery. *Rapid progression to acute renal failure* - While horseshoe kidney can be associated with an increased risk of complications (like UPJO, stones, infections), it does not inherently lead to **rapid progression to acute renal failure**. - Many individuals with a horseshoe kidney have **normal renal function** without significant clinical manifestations. *Kidneys are usually non-functional.* - The presence of a horseshoe kidney **does not typically mean the kidneys are non-functional**. - In most cases, both renal units of a horseshoe kidney are **functional**, although they may be at increased risk for complications that could impact function over time. *Increased risk of developing renal vein thrombosis* - There is **no established increased risk** of developing **renal vein thrombosis** specifically associated with horseshoe kidney. - The primary vascular anomalies associated with horseshoe kidney relate to the arterial supply and variations in the number and origin of renal arteries, not typically venous thrombosis.

Question 53: A 2-day-old infant dies of severe respiratory distress following a gestation complicated by persistent oligohydramnios. Upon examination at autopsy, the left kidney is noted to selectively lack cortical and medullary collecting ducts. From which of the following embryological structures do the cortical and medullary collecting ducts arise?

A. Mesonephros
B. Pronephros
C. Ureteric bud (Correct Answer)
D. Paramesonephric duct
E. Metanephric mesenchyme

Explanation: ***Ureteric bud*** - The **ureteric bud** is an outgrowth of the mesonephric duct that gives rise to the **collecting ducts**, major and minor calyces, renal pelvis, and ureter. - Absence or anomaly of the ureteric bud's development leads to conditions like **renal agenesis** or **renal hypoplasia**, which can cause oligohydramnios and Potter sequence due to insufficient urine production. *Mesonephros* - The **mesonephros** is a temporary kidney that functions early in development but largely degenerates. - Its tubules, in males, contribute to the **epididymis**, **vas deferens**, and **ejaculatory duct**. *Pronephros* - The **pronephros** is the first and most primitive kidney structure, forming early in embryonic development. - It rapidly **degenerates** and has no functional role in human development beyond inducing mesonephros development. *Paramesonephric duct* - The **paramesonephric (Müllerian) duct** is critical for the development of the female reproductive tract. - It forms the **fallopian tubes**, **uterus**, and **upper vagina**. *Metanephric mesenchyme* - The **metanephric mesenchyme** (or metanephric blastema) differentiates into the structures involved in urine filtration and initial processing. - This includes the **glomeruli**, Bowman's capsule, proximal convoluted tubules, loops of Henle, and distal convoluted tubules.

Question 54: A 23-year-old woman, gravida 2, para 1, at 26 weeks gestation comes to the physician for a routine prenatal visit. Physical examination shows a uterus consistent in size with a 26-week gestation. Fetal ultrasonography shows a male fetus with a thick band constricting the right lower arm; the limb distal to the constrictive band cannot be visualized. The most likely condition is an example of which of the following embryological abnormalities?

A. Agenesis
B. Disruption (Correct Answer)
C. Aplasia
D. Deformation
E. Malformation

Explanation: ***Disruption*** - A **disruption** is a morphological defect of an organ or a larger body region resulting from an **extrinsic breakdown** of a previously normal developmental process. - The **amniotic band syndrome**, causing the constricting band and absent distal limb, is a classic example of disruption due to **amniotic bands** entrapping fetal parts. *Agenesis* - **Agenesis** refers to the complete **absence of an organ** due to the absence of the primordial tissue from which it develops. - In this case, the limb was initially present but was subsequently damaged, which is not agenesis. *Aplasia* - **Aplasia** refers to the complete **absence of an organ** when the primordial tissue was present but failed to develop. - This differs from the scenario where a previously normally developing structure is destroyed by an extrinsic factor (disruption). *Deformation* - A **deformation** is an abnormality in form or position of a body part caused by **mechanical forces**, usually occurring in later fetal stages. - Examples include clubfoot due to intrauterine compression, but it does not involve the intrinsic destruction of tissue observed here. *Malformation* - A **malformation** is a **primary defect** in the development of an organ or tissue due to an intrinsic abnormal developmental process. - Examples include **cleft lip** or congenital heart defects; it is not due to an external disruptive force.

Question 55: A 35-year-old G2P1 delivers a boy in the 40th week of gestation. The pregnancy was uncomplicated. The newborn had Apgar scores of 7 and 9 at the 1st and 5th minutes, respectively. On physical examination, the newborn is noted to have a left-sided cleft lip. The hard palate and nose are normal. Which of the following statements describes the cause of the abnormality?

A. Failure of fusion of the left maxillary prominence and the medial nasal process of the frontonasal prominence (Correct Answer)
B. Failure of fusion of the left maxillary prominence and the lateral nasal process of the frontonasal prominence
C. Failure of development of the left maxillary prominence
D. Partial resorption of the first pharyngeal arch
E. Failure of development of the first pharyngeal arch

Explanation: ***Failure of fusion of the left maxillary prominence and the medial nasal process of the frontonasal prominence*** - **Cleft lip** results from the **failure of fusion** between the **maxillary prominence** and the **medial nasal process** during the 6th to 7th week of embryonic development. - The **medial nasal processes** form the philtrum of the upper lip, the primary palate, and the central part of the upper jaw, while the **maxillary prominences** form the lateral parts of the upper lip and the secondary palate. *Failure of fusion of the left maxillary prominence and the lateral nasal process of the frontonasal prominence* - The **lateral nasal processes** form the alae of the nose but do not directly contribute to the upper lip formation; therefore, their failure to fuse with the maxillary prominence would not cause a cleft lip. - Fusion issues involving the lateral nasal process are associated with anomalies of the nasal structures, not the lip itself. *Failure of development of the left maxillary prominence* - Complete failure of development of the maxillary prominence would lead to a more severe facial defect, often involving hypoplasia or absence of the maxilla and not just an isolated cleft lip. - Such a severe defect would likely impact other craniofacial structures more broadly than described. *Partial resorption of the first pharyngeal arch* - The **first pharyngeal arch** forms many structures of the face, including the maxilla and mandible, but cleft lip is a failure of fusion, not a resorption defect of a fully formed structure. - Resorption issues are typically associated with conditions like Treacher Collins syndrome, affecting bones and soft tissues derived from the arch. *Failure of development of the first pharyngeal arch* - Complete or significant failure of development of the **first pharyngeal arch** would result in severe facial malformations, including micrognathia, severe ear abnormalities, and hypoplasia of the maxilla and mandible. - A simple cleft lip, especially when the nose and hard palate are normal, does not typically point to a global failure of the first pharyngeal arch development.

Question 56: A 16-year-old boy comes to the physician because of a 1-week history of difficulty swallowing, a foreign body sensation at the back of his throat, and trouble breathing at night. He has just recovered from an upper respiratory tract infection that began 5 days ago. On questioning, he reports that he has had similar symptoms in the past each time he has had an upper respiratory tract infection. Physical examination shows a 3 x 2-cm, nontender, rubbery midline mass at the base of the tongue. His skin is dry and cool. An image of his technetium-99m pertechnetate scan is shown. Which of the following is the most likely underlying cause of this patient’s condition?

A. Chronic infection of the palatine and lingual tonsils
B. Ductal obstruction of the sublingual salivary glands
C. Arrested endodermal migration from pharyngeal floor (Correct Answer)
D. Persistent epithelial tract between the foramen cecum and thyroid isthmus
E. Failure of obliteration of the second branchial cleft

Explanation: ***Arrested endodermal migration from pharyngeal floor*** - This describes the embryological origin of a **lingually ectopic thyroid gland** (also known as a **lingual thyroid**), which results from the thyroid's failure to descend from its embryonic origin at the foramen cecum. - The symptoms of globus sensation, dysphagia, and trouble breathing, especially exacerbated during URI due to swelling of the ectopic thyroid tissue, alongside the midline tongue mass and positive **technetium-99m pertechnetate scan** (which highlights thyroid tissue), are highly indicative of this condition. *Chronic infection of the palatine and lingual tonsils* - While chronic tonsillitis can cause symptoms like dysphagia and foreign body sensation, it typically presents with recurrent sore throats, pus, and tender lymphadenopathy. - It would not explain a distinct midline mass at the base of the tongue that shows uptake on a technetium-99m scan, which specifically targets thyroid tissue. *Ductal obstruction of the sublingual salivary glands* - Obstruction of sublingual ducts (e.g., ranula) would typically present as a soft, bluish, fluid-filled mass in the floor of the mouth, not a rubbery midline mass at the base of the tongue. - This condition is unrelated to URIs and would not show uptake on a technetium-99m pertechnetate scan. *Persistent epithelial tract between the foramen cecum and thyroid isthmus* - This describes a **thyroglossal duct cyst**, which is typically a midline neck mass that moves with tongue protrusion. - While embryologically related to thyroid development, the mass in this patient is described as being at the "base of the tongue," not a mobile neck mass, and represents a completely ectopic thyroid gland rather than a cystic remnant of the duct. *Failure of obliteration of the second branchial cleft* - This leads to the formation of a **branchial cleft cyst**, which is usually found laterally in the neck, anterior to the sternocleidomastoid muscle. - It is not typically a midline mass at the base of the tongue and is generally not associated with symptoms of dysphagia or respiratory distress exacerbated by URIs in this specific manner.

Question 57: A 31-year-old G2P2 female at 40 weeks gestation presents to the hospital following a rush of water that came from her vagina. She is 4 cm dilated and 80% effaced. Fetal heart tracing shows a pulse of 155/min with variable decelerations. About 12 hours after presentation, she gives birth to a 6 lb 15 oz baby boy with APGAR scores of 8 and 9 at 1 and 5 minutes, respectively. Which of the following structures is responsible for inhibition of female internal genitalia?

A. Allantois
B. Syncytiotrophoblast
C. Leydig cells
D. Spermatogonia
E. Sertoli cells (Correct Answer)

Explanation: ***Sertoli cells*** - **Sertoli cells** in the male fetus produce **anti-Müllerian hormone (AMH)**, which is crucial for the regression of the **Müllerian ducts**. - The **Müllerian ducts** are the precursors to the female internal genitalia (uterus, fallopian tubes, and upper vagina), so their inhibition is essential for male sexual development. *Allantois* - The **allantois** is a fetal membrane involved in waste storage and gas exchange, contributing to the formation of the **urachus** and parts of the bladder. - It plays no direct role in the inhibition or development of internal genitalia. *Syncytiotrophoblast* - The **syncytiotrophoblast** is a layer of placental tissue that produces hormones vital for maintaining pregnancy, such as **human chorionic gonadotropin (hCG)**. - It is not involved in the sexual differentiation of the internal genitalia. *Leydig cells* - **Leydig cells** produce **testosterone**, which stimulates the development of the **Wolffian ducts** into male internal genitalia (epididymis, vas deferens, seminal vesicles). - While essential for male development, they do not inhibit female internal genitalia; that function belongs to Sertoli cells. *Spermatogonia* - **Spermatogonia** are germ cells that differentiate into spermatozoa in the adult male. - They are primordials of reproductive cells and do not have an endocrine role in embryonic sexual differentiation.

Question 58: A 23-year-old man comes to the physician for evaluation of decreased hearing, dizziness, and ringing in his right ear for the past 6 months. Physical examination shows multiple soft, yellow plaques and papules on his arms, chest, and back. There is sensorineural hearing loss and weakness of facial muscles bilaterally. His gait is unsteady. An MRI of the brain shows a 3-cm mass near the right internal auditory meatus and a 2-cm mass at the left cerebellopontine angle. The abnormal cells in these masses are most likely derived from which of the following embryological structures?

A. Surface ectoderm
B. Neural tube
C. Neural crest (Correct Answer)
D. Notochord
E. Mesoderm

Explanation: ***Neural crest*** - The patient's symptoms (bilateral sensorineural hearing loss, facial weakness, unsteady gait, central masses) along with cutaneous lesions (soft, yellow plaques) are highly suggestive of **Neurofibromatosis type 2 (NF2)**. - NF2 is characterized by **vestibular schwannomas** (acoustic neuromas) and other CNS tumors, which are derived from **Schwann cells**. Schwann cells, along with melanocytes and other peripheral nervous system components, originate from the **neural crest**. *Surface ectoderm* - The surface ectoderm forms structures such as the **epidermis**, hair, nails, and anterior pituitary. - While the skin lesions are present, the primary tumors (schwannomas) are not derived from the surface ectoderm. *Neural tube* - The neural tube gives rise to the **central nervous system** (brain and spinal cord) and motor neurons. - While the tumors affect the brain and cranial nerves, the specific cell type forming schwannomas (Schwann cells) does not originate directly from the neural tube. *Notochord* - The notochord induces the formation of the neural tube and eventually degenerates, contributing to the **nucleus pulposus** of the intervertebral discs. - It is not involved in the pathogenesis or cellular origin of schwannomas. *Mesoderm* - The mesoderm gives rise to connective tissues, blood, bone, muscle, and most internal organs. - While some tumors can have mesodermal origins, schwannomas are neuroectodermal in origin.

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