Embryology — Past Paper Questions & Answers

Question 1

A newborn male infant is examined. There is a palpable right testis, but no left testis, in the scrotal sac. The infant has no other abnormalities noted. Which of the following is the most likely abnormality involving the gonads of this infant?

Accepted Answer

Incomplete descent — Cryptorchidism is one of the most common problems involving the testis. Descent may be delayed. It can sometimes be chemically induced. Surgical correction may be employed. If not corrected, the affected gonad will not have spermatogenesis, and there will be an increased risk for carcinoma. ---

Question 2

Following ovulation and fertilization of an ovum, a blastocyst develops. Growth and differentiation occurs following implantation. Which of the following differentiated tissues will develop into the placenta?

Accepted Answer

Trophoblast — Early in development, cells in the blastocyst destined to become the placenta become trophoblasts that line a blastocystic cavity, with an inner cell mass composed of embryoblasts destined to become the fetus. Hence, the placenta is a fetal structure, not a maternal structure. ---

Question 3

A 5-year-old child fatigues easily. On examination a machinery-like murmur is auscultated over his chest. Echocardiography shows shunting of blood from the aorta to the pulmonary artery. An abnormality involving which of the following persistent embryologic structures is most likely present in this boy?

Accepted Answer

Ductus arteriosus — The ductus arteriosus is normally open in fetal life to provide a shunt of blood from the pulmonary circulation to the systemic circulation. It should close soon after birth. If it does not close, there is a left-to-right shunt. ---

Question 4

An infant born at term is examined, and there is abnormal fusion of the 3rd, 4th, and 5th digits of the left hand. A radiograph of the left hand shows transformation of metacarpals I, II, III, and V to short, carpal-like bones. A mutation involving which of the following genes most likely led to these findings?

Accepted Answer

HOXD13 — This gene is in the homeobox family of genes producing transcription factors that control morphogenesis, including limb development. Mutations may lead to lack of separation (syndactyly). ---

Question 5

An infant born at term has a radiograph taken. No congenital abnormalities are noted. The upper heart borders are not well visualized. Which of the following structures normally present is most likely to obscure the heart borders in this infant's radiograph?

Accepted Answer

Thymus — The thymus is in the anterior mediastinum and is proportionately larger in a newborn than later in life, and is busily schooling T lymphocytes, so it has a soft tissue density similar to the heart that lies posterior to it. In adults, the thymus appears on radiographs as only a small fatty remnant. ---

Question 6

A secondary oocyte is developing normally. At which of the following times does the second maturation division of meiosis become completed?

Accepted Answer

At fertilization — When the sperm penetrates the ovum, the process of meiosis is completed, with expulsion of the last polar body. ---

Question 7

During the 3rd week of embryonic development, splanchnic mesoderm differentiates into blood islands with angioblasts. In what developing tissue site does this process occur?

Accepted Answer

Heart — The first significant amount of red blood cell production occurs in paired heart tubes, but quickly moves to spleen, then liver for most of 2nd and 3rd trimester, then marrow in 3rd trimester and beyond. ---

Question 8

An ovum is fertilized. After 3 weeks a blastocyst implanted on the endometrium has an inner embryoblast and an outer trophoblast. Which of the following is the next step in forming a 3-layered embryonic disk?

Accepted Answer

Gastrulation — The 3 germ layers form in the process of gastrulation in the 3rd week of embryonic development. ---

Question 9

A 55-year-old man has noted sharp pain in his right lower extremity for the past 2 months. MR imaging of his spine shows impingement on a spinal nerve root by a herniated structure located between L5 and S1. From which of the following embryonic derivatives does this herniated structure most likely arise?

Accepted Answer

Notochord — The intervertebral disk has a central nucleus pulposus that is a notochordal remnant that can herniate to produce spinal nerve compression. ---

Question 10

In the developing embryo, pharyngeal arches are formed. The second (branchial) arch gives rise to the stapedius, orbicularis oris, and posterior belly of the digastric muscles. Which of the following cranial nerves is also derived from this arch?

Accepted Answer

III — CN VII is the facial nerve, innervating muscles of facial expression. ---

Question 11

In the normally developing embryo, aortic arches are apparent by the end of the fourth week. Which of the following arteries is derived from the third pair of aortic arches?

Accepted Answer

Carotid — The common carotids and the first part of the internal carotids are derivatives of the 3rd aortic arch. ---

Question 12

A fetus at 18 weeks gestation is normally formed and appropriately sized. No fetal or placental anomalies are present. Which of the following parts of the fetal vasculature is most likely to have the greatest oxygen concentration?

Accepted Answer

Umbilical vein — The umbilical vein is carrying oxygenated blood from the placenta into the fetus. ---

Question 13

An embryonic eye is developing normally. A retina is present on the posterior aspect of the globe. What is the embryologic origin of the portion of retina containing nerve cell bodies?

Accepted Answer

Diencephalon — The neural retina is a diencephalic structure. The telencephalon (cerebral hemispheres) and the diencephalon (including thalamus and hypothalamus) are derivatives of the prosencephalon. ---

Question 14

A newborn examination is performed. The tympanic membranes are visualized bilaterally. What is the embryologic origin of the structure that forms the canal connecting the external ear to the tympanic membrane?

Accepted Answer

First pharyngeal cleft — The external auditory canal is a derivative of the first pharyngeal cleft (groove). ---

Question 15

A 2-year-old child has difficulty eating and drinking. On examination, there is a lateral cleft that involves the right upper lip. This defect is most likely due to failure in development and fusion of which of the following?

Accepted Answer

Medial nasal prominences and maxillary prominences F. Medial nasal prominences and secondary palate — The appearance of a cleft lip is described. Most are positioned lateral to midline, because most cleft lips result from failure of fusion of a medial nasal prominence and a maxillary prominence. The medial nasal prominences are on the inner edge of the nasal pits. ---

Question 16

A 20-year-old woman is unable to conceive and undergoes an infertility workup. A pelvic ultrasound reveals a bicornuate uterus. Which of the following is the most likely explanation for her anomalous uterus?

Accepted Answer

Failure of the paramesonephric ducts to fuse — The paramesonephric (Mullerian) ducts form the uterus, and failure of fusion in the upper portion leaves two cavities, not one. Uterine anomalies account for a small number of cases of infertility, because implantation or growth of the developing embryo is affected. ---

Question 17

A newborn boy infnat is noted to be dribbling urine. On examination he has an abnormal opening extending from the meatus to the ventral-midpoint of his penis. Abnormal development of which of the following structures is most likely responsible for his urinary problem?

Accepted Answer

Urethral fold F. Urogenital sinus — A condition called hypospadias is described here, an anomaly of closure of the penile urethra. Most cases are a partial defect. ---

Question 18

In a developing embryo, neural crest cells form adjacent to the neural tube. These neural crest cells then undergo migration. Which of the following structures is derived from these migrating cells?

Accepted Answer

Dorsal root ganglion — Neural crest develops lateral to the notochord, and neural crest cells migrate widely. They give rise to ganglia and to a variety of neuroendocrine cells in various organs. ---

Question 19

A newborn infant boy is noted to have an abnormality involving his perineum and scrotum. There is failure of midline fusion in the urogenital region. Chromosome analysis shows his karyotype is 46, XY. Which of the following is most likely responsible for his condition?

Accepted Answer

Deficient androgen levels in utero — Testosterone is converted to dihydrotestosterone that induces labioscrotal swellings to fuse and become the scrotum. ---

Question 20

The two superior and two inferior parathyroid glands are found adjacent to the posterior aspect of the thyroid gland. From which of the following are these parathyroid glands derived in embryologic development?

Accepted Answer

Pharyngeal pouches — The fourth pouches give rise to the superior pair of parathyroid glands and the parafollicular C cells within the thyroid; the third pouch gives rise to the inferior parathyroids and the non-lymphoid cells of the thymus. ---

Question 21

A 31-year-old woman has noted no fetal movement by 16 weeks gestation. A fetal ultrasound shows the fetal head contains a brain with thin cortex, single enlarged ventricle, and fused thalami. Which of the following developing regions of the brain is most likely abnormal in this fetus?

Accepted Answer

Prosencephalon — Holoprosencephaly is described here. The telencephalon (cerebral hemispheres) and the diencephalon (including thalamus and hypothalamus) are derivatives of the prosencephalon. ---

Question 22

During normal embryonic development, anterior-posterior differentiation occurs. By the end of the fifth week, dorsomedial somitic cells give rise to myotomes forming the epimere. Which of the following muscles is most likely derived from these cells?

Accepted Answer

Erector spinae — The epimere consists of myotomes derived from dorsomedial somitic cells that form dorsal (posterior in humans) back muscles. ---

Question 23

In the developing embryo, limb buds form along an anterior-posterior axis as directed by a zone of polarizing activity (ZPA) in the mesenchyme. A product of which of the following genes is most important in signaling by the ZPA?

Accepted Answer

Sonic hedgehog — Shh plays a role in this induction of limb bud development. The Shh protein creates a gradient effect upon developing tissues, near to far, guiding proliferation, differentiation, and movement of cells. ---

Question 24

In the developing male embryo, primordial germ cells migrate and contribute to formation of gonads. These cells remain dormant until stimulated to differentiate. When do these primordial germ cells differentiate to spermatogonia?

Accepted Answer

Puberty F. 20 years of age — At puberty hormonal changes occur that induce differentiation of germ cells so that spermatogenesis and fertility occur. ---

Question 25

Following fertilization of the ovum, the ovarian corpus luteum continues to produce progesterone. If the corpus luteum regresses in the first trimester, abortion occurs. Which of the following hormones maintains this capability of the corpus luteum?

Accepted Answer

Human chorionic gonadotropin — HCG is produced by the trophoblastic cells of the imlanted blastocyst that differentiate into the placenta. If fertilization and implantation do not occur, then no HCG is produced and the corpus luteum regresses to a corpus albicans. Menstruation occurs, and the endometrial hormonal cycle repeats. ---

Question 26

Following coitus, spermatozoa migrate to the fallopian tubes. Within the tubes, the acrosomal head of spermatozoa undergoes reduction of glycoprotein coating with increasing permeability to calcium. Which of the following functions of spermatozoa is facilitated by this process?

Accepted Answer

Fertilization — This process is called capacitation, and provides greater interactivity between the sperm and the ovum for penetration to occur. ---

Question 27

A 22-year-old woman has noted no fetal movement at 17 weeks gestation. A screening fetal ultrasound shows a midline abdominal wall defect, with bowel contents contained within a membranous sac. Her serum alpha-fetoprotein is increased. Which of the following abnormalities is most likely present in her fetus?

Accepted Answer

Omphalocele — A midline defect involving the umbilicus and covered by a sac is called an omphalocele. Abdominal contents such as bowel can herniate into the sac. Some omphaloceles are associated with chromosomal anomalies such as trisomy 18 (Edwards syndrome). ---

Question 28

During week 3 of embryonic development, the mesoderm becomes organized into separate tissue regions. An amnionic cavity forms from amnioblasts, derivatives of the epiblast. Which of the following portions of mesoderm surrounds this amnionic cavity?

Accepted Answer

Extraembryonic — The extraembryonic somatic mesodermal layer lines trophoblast and surrounds the amnionic cavity of the developing embryo. The extraembryonic splanchnic mesoderm surrounds the umbilical vesicle, a temporary structure. ---

Question 29

During embryonic development, neural crest cells migrate to the skin. Some of these cells differentiate to produce melanin from tyrosine. These cells may be destroyed in a patchy distribution by an autoimmune process. What is the most likely condition to result from this autoimmune process?

Accepted Answer

Vitiligo — Patchy loss of pigmentation is typically the result of an autoimmune process directed at melanocytes. ---

Question 30

A neonate born at term is noted at 1 week of age to exhibit cyanosis. The oxygen saturation is 80%. Which of the following abnormalities in cardiac development best explains these findings?

Accepted Answer

Conotruncal ridges fail to fuse — Cyanosis suggest right-to-left shunting that can happen with tetralogy of Fallot, transposition of the great vessels, and truncus arteriosis. The latter results when the conotruncal ridges fail to fuse so that there is mixing of venous pulmonic trunk and oxygenated aortic blood. ---

Question 31

A newborn boy infant born at term is noted to have clear fluid leaking from his umbilical region. In contrast, there is minimal urine output via the penis onto the diaper. The pH of the leaking fluid is 6.5. Which of the following is the most likely abnormality in this boy?

Accepted Answer

Bladder exstrophy — Persistence of the allantois may leave a urachal remnant superior to the bladder, either a cyst or a fistula from bladder to umbilicus. Urine can leak through a fistulous tract. Urine typically is clear (no cells and minimal protein) with pH below 7. Small intestinal contents are cloudy and alkaline in pH. ---

Question 32

A 24-year-old primigravida has felt no fetal movement at 16 weeks gestation. A screening fetal ultrasound shows anhydramnios, poorly formed pelvic bones, and no recognizable lower extremities. An abnormality involving which of the following structures was most likely present with these findings?

Accepted Answer

Lumbosacral spine — The condition described is caudal regression syndrome. Limb buds did form, but compete development was restricted. Pelvic structures such as the lower urinary tract may also be affected. ---

Question 33

Cells from the dorsolateral portion of a somite form a ventral hypomere with myotomes. Which of the following portions of the nervous system form nerves that migrate with the myotomes during development?

Accepted Answer

Ventral primary ramus — The ventral primary ramus has nerve fibers that innervate the hypomeres. ---

Question 34

A 44-year-old woman gives birth at 34 weeks gestation to a low birth weight male infant. On examination there is brachycephaly and uplanting palpebral fissues. There is a cleft in the iris bilaterally. Which of the following terms describes the defect in the iris of this infant?

Accepted Answer

Coloboma F. Cyclopia — Failure of closure of the choroid fissure leads to coloboma. It may involve just the iris, or the defect may extend all the way to the optic nerve. It may be an isolated anomaly, or part of multiple congenital anomalies, as in this case with findings suggestive of trisomy 21. ---

Question 35

A 1-month-old infant has failure to thrive. On examination a heart murmur is auscultated over the chest. No cyanosis is noted. Echocardiography shows a left-to-right shunt just above the muscular interventricular septum. An abnormality of which of the following structures most likely produced this infant's defect?

Accepted Answer

Endocardial cushion — The endocardial cushions contribute to the atrioventricular valves and the septae,including the membranous portion of the interventricular septum. This is the most common form of ventricular septal defect (VSD), which is the most common heart defect in liveborns. ---

Question 36

A 19-year-old primigravida notes minimal fetal movement at 17 weeks gestation. The maternal serum alpha-fetoprotein is not increased. Fetal ultrasound shows enlargement of lateral ventricles of the brain. Which of the following abnormalities is the most likely cause?

Accepted Answer

Aqueduct of Sylvius stenosis — Aqueductal stenosis prevents passage of cerebrospinal fluid from the lateral ventricles and third ventricle to the fourth ventricle, leading to an obstructive, non-communicating hydrocephalus. Since the AFP is not elevated, there is no open defect. ---

Question 37

In the normally developing embryo, the mesonephric duct gives rise to the ureteric bud. The excretory system develops from migration of the ureteric bud into metanephric tissue. Which of the following renal components has aquaporin receptors and is derived from the ureteric bud?

Accepted Answer

Collecting ducts — The ureteric bud organizes the metanephros, and develops as far as the distal part of the distal convoluted tubules. The collecting ducts are sensitive to antidiuretic hormone (ADH) to regulate free water loss. ---

Question 38

At 24 weeks gestation, a developing tooth had multiple layers of tissue. Which of the following layers contains a vascular component?

Accepted Answer

Pulp — The pulp is the innermost layer of the tooth and contains vessels and nerves. ---

Question 39

A 4-year-old child has had nausea and vomiting for the past year. She exhibits early satiety with meals. An abdominal ultrasound is performed and shows duodenal narrowing. Abnormal migration of which of the following structures most likely occurred in this girl when she was in utero?

Accepted Answer

Ventral pancreatic bud — The dorsal and ventral pancreatic buds rotate and fuse to form the exocrine pancreas. The two parts of the ventral bud may rotate in opposite directions to surround the duodenum and produce obstruction. This condition is called annular pancreas. ---

Question 40

A neonate born at term after an uncomplicated pregnancy is noted to pass pale coloured stool and dark urine. The infant becomes progressively jaundiced in the next 2 days. On examination the infant is at the 50th percentile for weight. Which of the following developmental abnormalities most likely occurred in utero?

Accepted Answer

Biliary atresia — Atretic ducts cannot pass bile. the bile will back up into liver and spill into blood. Hepatic cirrhosis ensues. Extrahepatic biliary atresia is potentially correctible by surgery. Intrahepatic biliary atresia requires liver transplantation. ---

Question 41

Following birth, circulation from placenta to fetus ceases. Which of the following fetal structures that conducts blood to the inferior vena cava becomes atretic in postnatal life?

Accepted Answer

Ductus venosus — The ductus venosus extends from

Embryology — Past Paper Questions & Answers

Questions, Answers & Explanations