USMLE Step 1 Embryology Practice Questions

Fertilization, the fusion of sperm and egg to form a zygote, typically occurs in which part of the female reproductive tract?

• (A) Ampulla of the fallopian tube (uterine tube)
• (B) Uterus
• (C) Ovary
• (D) Cervix

During the first week of embryonic development, the zygote undergoes rapid mitotic divisions without significant growth, forming a solid ball of cells called a morula. These divisions are known as:

• (A) Cleavage
• (B) Gastrulation
• (C) Neurulation
• (D) Organogenesis

Implantation of the blastocyst into the uterine wall typically begins around which day after fertilization?

• (A) Day 6-7
• (B) Day 1
• (C) Day 14
• (D) Day 21

Gastrulation, a critical process occurring during the third week of development, establishes the three primary germ layers. Which structure forms first during gastrulation and serves as the site where epiblast cells migrate inward?

• (A) Primitive streak
• (B) Notochord
• (C) Neural tube
• (D) Somites

The three primary germ layers established during gastrulation give rise to all tissues and organs in the body. Which germ layer primarily forms the nervous system (brain, spinal cord) and the epidermis of the skin?

• (A) Ectoderm
• (B) Mesoderm
• (C) Endoderm
• (D) Trophoblast

The notochord, a transient midline structure derived from mesoderm, plays a crucial role in inducing the overlying ectoderm to differentiate into the neural plate. This process is known as:

• (A) Neural induction
• (B) Gastrulation
• (C) Somitogenesis
• (D) Vasculogenesis

Failure of the anterior (rostral) neuropore to close properly during neurulation (around day 25) results in which severe congenital anomaly?

• (A) Anencephaly
• (B) Spina bifida occulta
• (C) Meningocele
• (D) Myelomeningocele

Which germ layer gives rise to muscle tissue (skeletal, smooth, cardiac), connective tissues (bone, cartilage, blood), the cardiovascular system, and the urogenital system?

• (A) Mesoderm
• (B) Ectoderm
• (C) Endoderm
• (D) Neural crest

Somites, blocks of paraxial mesoderm formed in segments along the neural tube, differentiate into three components. Which component gives rise to the vertebrae and ribs?

• (A) Sclerotome
• (B) Dermatome
• (C) Myotome
• (D) Intermediate mesoderm

The pharyngeal (branchial) arches are transient structures in the embryonic neck region that contribute significantly to the formation of the head and neck structures. Which cranial nerve is associated with the first pharyngeal arch?

• (A) Trigeminal nerve (CN V)
• (B) Facial nerve (CN VII)
• (C) Glossopharyngeal nerve (CN IX)
• (D) Vagus nerve (CN X)

The heart begins as a simple tube that undergoes complex looping and septation to form the four-chambered structure. Which embryonic structure gives rise to the smooth-walled parts of the right and left atria?

• (A) Sinus venosus
• (B) Primitive atrium
• (C) Primitive ventricle
• (D) Bulbus cordis

Which embryonic vessel carries oxygenated blood from the placenta to the fetus?

• (A) Umbilical vein
• (B) Umbilical arteries (two)
• (C) Ductus arteriosus
• (D) Ductus venosus

The kidneys develop through three successive systems: pronephros, mesonephros, and metanephros. Which of these systems forms the permanent, functional kidney in humans?

• (A) Metanephros
• (B) Pronephros
• (C) Mesonephros
• (D) Cloaca

DiGeorge syndrome is characterized by craniofacial anomalies, congenital heart defects (conotruncal region), thymic hypoplasia/aplasia (leading to immunodeficiency), and hypoparathyroidism (leading to hypocalcemia). This syndrome results from abnormal development of which pharyngeal pouches?

• (A) Third and Fourth pharyngeal pouches
• (B) First and Second pharyngeal pouches
• (C) Second and Third pharyngeal pouches
• (D) Fifth pharyngeal pouch

The lungs develop as an outgrowth (respiratory diverticulum or lung bud) from which part of the primitive gut tube?

• (A) Foregut
• (B) Midgut
• (C) Hindgut
• (D) Yolk sac

Which embryonic structure persists in the adult as the nucleus pulposus of the intervertebral discs?

• (A) Notochord
• (B) Sclerotome
• (C) Neural tube
• (D) Primitive streak

Failure of the migration of neural crest cells into the distal colon results in the absence of enteric ganglia (Auerbach's and Meissner's plexuses) in that segment. This leads to functional obstruction and dilation of the proximal colon. What is this condition called?

• (A) Hirschsprung disease (Congenital aganglionic megacolon)
• (B) Duodenal atresia
• (C) Meckel's diverticulum
• (D) Intussusception

The fusion of the two medial nasal prominences during facial development forms which structures?

• (A) Intermaxillary segment (philtrum of upper lip, primary palate, four incisor teeth)
• (B) Cheeks and lateral parts of upper lip
• (C) Lower lip and mandible
• (D) Nasal bridge

Which fetal circulatory shunt allows blood to bypass the pulmonary circulation by flowing directly from the pulmonary artery into the aorta?

• (A) Ductus arteriosus
• (B) Ductus venosus
• (C) Foramen ovale
• (D) Umbilical vein

The gonads (testes and ovaries) initially develop as indifferent structures. The presence of which gene, typically located on the Y chromosome, directs the indifferent gonad to differentiate into a testis?

• (A) SRY (Sex-determining Region Y) gene
• (B) SOX9 gene
• (C) WNT4 gene
• (D) DAX1 gene

Which extraembryonic membrane forms the fetal contribution to the placenta and is involved in nutrient and gas exchange between mother and fetus?

• (A) Chorion
• (B) Amnion
• (C) Yolk sac
• (D) Allantois

A newborn infant presents with cyanosis and respiratory distress. Echocardiogram reveals that the aorta arises from the right ventricle and the pulmonary artery arises from the left ventricle. This congenital heart defect, resulting from failure of the aorticopulmonary septum to spiral correctly during development, is known as:

• (A) Transposition of the Great Arteries (TGA)
• (B) Tetralogy of Fallot (TOF)
• (C) Persistent Truncus Arteriosus
• (D) Total Anomalous Pulmonary Venous Return (TAPVR)

The umbilical cord typically contains which vessels?

• (A) Two umbilical arteries and one umbilical vein
• (B) One umbilical artery and one umbilical vein
• (C) Two umbilical arteries and two umbilical veins
• (D) One umbilical artery and two umbilical veins

Which structure develops from the fusion of the paramesonephric (Müllerian) ducts in females?

• (A) Uterus, cervix, and upper part of the vagina
• (B) Ovaries
• (C) Lower part of the vagina and external genitalia
• (D) Epididymis, ductus deferens, seminal vesicles

Meckel's diverticulum, the most common congenital anomaly of the gastrointestinal tract, results from the persistence of which embryonic structure?

• (A) Vitelline duct (omphalomesenteric duct)
• (B) Allantois
• (C) Notochord
• (D) Urachus

The process by which the flat, trilaminar embryonic disc folds laterally and craniocaudally to form a cylindrical embryo is known as:

• (A) Embryonic folding
• (B) Gastrulation
• (C) Neurulation
• (D) Cleavage

Which cells migrate from the yolk sac endoderm to the developing gonadal ridges and eventually differentiate into sperm or oocytes?

• (A) Primordial germ cells (PGCs)
• (B) Sertoli cells
• (C) Leydig cells
• (D) Granulosa cells

Cleft lip results from the failure of fusion between which facial prominences?

• (A) Maxillary prominence and Medial nasal prominence
• (B) Lateral nasal prominence and Medial nasal prominence
• (C) Mandibular prominences
• (D) Maxillary prominence and Lateral nasal prominence

The foramen ovale is an opening in the interatrial septum of the fetal heart that allows oxygenated blood from the inferior vena cava to flow directly from the right atrium to the left atrium, bypassing the pulmonary circulation. What structure normally forms from the closure of the foramen ovale after birth?

• (A) Fossa ovalis
• (B) Ligamentum arteriosum
• (C) Ligamentum venosum
• (D) Median umbilical ligament

Which pharyngeal arch gives rise to the muscles of facial expression, the stapes bone, styloid process, stylohyoid ligament, and the lesser horn/upper body of the hyoid bone?

• (A) Second pharyngeal arch (Hyoid arch)
• (B) First pharyngeal arch (Mandibular arch)
• (C) Third pharyngeal arch
• (D) Fourth and Sixth pharyngeal arches

The development of the diaphragm involves the fusion of several embryonic components. Which structure forms the central tendon of the diaphragm?

• (A) Septum transversum
• (B) Pleuroperitoneal membranes
• (C) Dorsal mesentery of the esophagus
• (D) Muscular ingrowth from body wall

Potter sequence (or syndrome) refers to the typical physical features (pulmonary hypoplasia, limb deformities, flattened facies) of a fetus caused by markedly deficient amniotic fluid (oligohydramnios). What is the most common underlying cause of severe oligohydramnios leading to Potter sequence?

• (A) Bilateral renal agenesis (failure of kidneys to develop)
• (B) Duodenal atresia
• (C) Tracheoesophageal fistula
• (D) Congenital diaphragmatic hernia

The thyroid gland develops from an endodermal thickening in the floor of the primitive pharynx and descends into the neck. Which transient structure connects the developing thyroid to its origin at the base of the tongue?

• (A) Thyroglossal duct
• (B) Foramen cecum
• (C) Ultimobranchial body
• (D) Pharyngeal pouch 3

Which signaling pathway plays a critical role in establishing the left-right asymmetry of the body plan during embryonic development?

• (A) Nodal signaling pathway
• (B) Wnt signaling pathway
• (C) Hedgehog signaling pathway
• (D) Notch signaling pathway

The formation of the neural tube from the neural plate is called neurulation. Which cell population arises from the lateral edges (neural folds) of the neural plate as the tube closes and migrates extensively throughout the embryo to form diverse cell types?

• (A) Neural crest cells
• (B) Somitic cells
• (C) Endodermal cells
• (D) Mesodermal cells

What is the developmental origin of the lens of the eye?

• (A) Surface ectoderm (Lens placode)
• (B) Neuroectoderm (Optic cup)
• (C) Mesoderm
• (D) Neural crest cells

During limb development, which signaling center, located at the distal tip of the limb bud (apical ectodermal ridge - AER), is essential for maintaining outgrowth along the proximal-distal axis?

• (A) Apical Ectodermal Ridge (AER)
• (B) Zone of Polarizing Activity (ZPA)
• (C) Progress Zone (PZ)
• (D) Somites

The pancreas develops from two buds (dorsal and ventral) arising from the foregut endoderm. Which part of the pancreas is primarily derived from the ventral pancreatic bud?

• (A) Uncinate process and inferior part of the head
• (B) Body and tail
• (C) Superior part of the head
• (D) Islets of Langerhans only

Teratogens are agents that can cause birth defects if exposure occurs during critical periods of development. Which of the following is a well-known teratogen associated with neural tube defects (like spina bifida) if taken during early pregnancy?

• (A) Valproic acid (Antiepileptic drug)
• (B) Vitamin A (in normal dietary amounts)
• (C) Folic acid supplementation
• (D) Acetaminophen

Which embryonic membrane encloses the fluid-filled cavity that surrounds and protects the developing embryo/fetus?

• (A) Amnion
• (B) Chorion
• (C) Yolk sac
• (D) Allantois