Somite formation and derivatives US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Somite formation and derivatives. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Somite formation and derivatives US Medical PG Question 1: A 10-year-old boy is brought to the physician by his parents for a follow-up examination. He has had a short stature since birth and underwent diagnostic testing. Genetic analyses showed a gain of function mutation in the fibroblast growth factor receptor 3 (FGFR3) gene. He has met all developmental milestones. He is at the 10th percentile for height and 90th percentile for weight. Which of the following additional findings is most likely on physical examination?
- A. Frontal bossing (Correct Answer)
- B. Absent collar bones
- C. Long extremities
- D. Blue sclerae
- E. Small genitalia
Somite formation and derivatives Explanation: ***Frontal bossing***
- A gain of function mutation in the **FGFR3 gene** is characteristic of **achondroplasia**, which is a common cause of short stature.
- **Frontal bossing** (prominent forehead) and midface hypoplasia are classical craniofacial features seen in individuals with achondroplasia.
*Absent collar bones*
- **Absent or hypoplastic clavicles** (collar bones) are a hallmark feature of **cleidocranial dysostosis**, a genetic disorder caused by mutations in the *RUNX2* gene, not typically associated with FGFR3 mutations.
- This condition also presents with delayed closure of fontanelles and dental anomalies, which are not suggested by the patient's presentation.
*Long extremities*
- Patients with achondroplasia characteristically have **rhizomelic short stature**, meaning the **proximal limbs** (femur and humerus) are disproportionately shorter than the trunk, leading to short extremities overall, not long ones.
- Long extremities are characteristic of conditions like **Marfan syndrome**, which has distinct genetic and clinical features.
*Blue sclerae*
- **Blue sclerae** are a common finding in **osteogenesis imperfecta**, a genetic disorder characterized by bone fragility due to defects in type I collagen synthesis.
- This condition is not caused by FGFR3 mutations and typically presents with recurrent fractures and other connective tissue abnormalities.
*Small genitalia*
- **Micropenis** (small genitalia) can be associated with various endocrine disorders, such as **hypogonadism** or congenital adrenal hyperplasia, or certain genetic syndromes.
- It is not a characteristic feature of achondroplasia or FGFR3-related skeletal dysplasias.
Somite formation and derivatives US Medical PG Question 2: 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
Somite formation and derivatives 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.
Somite formation and derivatives US Medical PG Question 3: 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
Somite formation and derivatives 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.
Somite formation and derivatives US Medical PG Question 4: 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
Somite formation and derivatives 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.
Somite formation and derivatives US Medical PG Question 5: 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
Somite formation and derivatives 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.
Somite formation and derivatives US Medical PG Question 6: An MRI of a patient with low back pain reveals compression of the L5 nerve root. Which of the following muscles would most likely show weakness during physical examination?
- A. Tibialis posterior
- B. Tibialis anterior (Correct Answer)
- C. Gastrocnemius
- D. Quadriceps femoris
Somite formation and derivatives Explanation: ***Tibialis anterior***
- The **L5 nerve root** primarily innervates muscles responsible for **dorsiflexion** of the foot, with the **tibialis anterior** being the primary dorsiflexor.
- Weakness of the tibialis anterior would manifest as difficulty lifting the front of the foot, potentially leading to a **foot drop** gait.
*Tibialis posterior*
- The **tibialis posterior** is primarily innervated by the **tibial nerve** (S1-S2) and is responsible for **plantarflexion** and **inversion** of the foot.
- Weakness in this muscle would not be the most likely presentation of L5 nerve root compression.
*Gastrocnemius*
- The **gastrocnemius** muscle is primarily innervated by the **tibial nerve** (S1-S2) and is a powerful **plantarflexor** of the foot.
- Weakness in this muscle would indicate an S1 or S2 nerve root issue, not typically L5.
*Quadriceps femoris*
- The **quadriceps femoris** is innervated by the **femoral nerve**, predominantly originating from the **L2, L3, and L4 nerve roots**.
- Weakness would manifest as difficulty extending the knee, which is not characteristic of L5 compression.
Somite formation and derivatives US Medical PG Question 7: A 26-year-old G1P0 woman comes to her maternal and fetal medicine doctor at 15 weeks of gestation in order to be evaluated for fetal developmental abnormalities. Her family has a history of congenital disorders leading to difficulty walking so she was concerned about her child. Amniocentesis shows normal levels of all serum proteins and circulating factors. Despite this, the physician warns that there is a possibility that there may be a neural tube abnormality in this child even though the normal results make it less likely. If this child was born with a neural tube closure abnormality, which of the following findings would most likely be seen in the child?
- A. Protrusion of the meninges and spinal cord through a bony defect
- B. Protrusion of the meninges through a bony defect
- C. Spinal cord able to be seen externally
- D. Tuft of hair or skin dimple on lower back (Correct Answer)
- E. Absence of the brain and calvarium
Somite formation and derivatives Explanation: ***Tuft of hair or skin dimple on lower back***
- This finding, particularly a **tuft of hair**, **skin dimple**, or **subcutaneous lipoma** on the lower back, is characteristic of **spina bifida occulta**.
- **Spina bifida occulta** is the least severe form of neural tube defect, where there is a bony defect in the vertebrae but the spinal cord and meninges remain within the spinal canal and are not externally evident.
*Protrusion of the meninges and spinal cord through a bony defect*
- This describes a **myelomeningocele**, which is a more severe form of spina bifida where the **spinal cord** and **meninges** protrude through a bony defect.
- Myelomeningocele typically presents with a visible sac on the back containing neural tissue, often leading to neurological deficits.
*Protrusion of the meninges through a bony defect*
- This describes a **meningocele**, where only the **meninges** protrude through a defect in the vertebral column, forming a fluid-filled sac.
- While it involves a visible sac, it does not contain neural tissue, and neurological symptoms are often absent or less severe compared to myelomeningocele.
*Spinal cord able to be seen externally*
- This is characteristic of **myeloschisis** or **rachischisis**, the most severe open neural tube defects where the **spinal cord** is open and exposed to the environment.
- This condition is often incompatible with life or leads to profound neurological impairment.
*Absence of the brain and calvarium*
- This describes **anencephaly**, a severe neural tube defect resulting from failure of closure of the anterior neural tube.
- Anencephaly is a lethal condition where the forebrain and cranial vault are absent, which is distinctly different from a spinal defect.
Somite formation and derivatives US Medical PG Question 8: 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
Somite formation and derivatives 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.
Somite formation and derivatives US Medical PG Question 9: 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
Somite formation and derivatives 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.
Somite formation and derivatives US Medical PG Question 10: 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
Somite formation and derivatives 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.
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