Embryological basis of dermatomes US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Embryological basis of dermatomes. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Embryological basis of dermatomes US Medical PG Question 1: 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
Embryological basis of dermatomes 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.
Embryological basis of dermatomes US Medical PG Question 2: A 68-year-old man presents to his primary care physician complaining of a bulge in his scrotum that has enlarged over the past several months. He is found to have a right-sided inguinal hernia and undergoes elective hernia repair. At his first follow-up visit, he complains of a tingling sensation on his scrotum. Which of the following nerve roots communicates with the injured tissues?
- A. S1-S3
- B. L1-L2 (Correct Answer)
- C. S2-S4
- D. L4-L5
- E. L2-L3
Embryological basis of dermatomes Explanation: ***L1-L2***
- The **ilioinguinal nerve** and **genitofemoral nerve**, which are commonly injured during inguinal hernia repair, arise from the **L1 and L2 spinal nerves**.
- These nerves provide sensory innervation to the **scrotum**, **inguinal region**, and **medial thigh**, explaining the patient's tingling sensation.
*S1-S3*
- These nerve roots typically contribute to the **sciatic nerve** and innervate the posterior thigh, leg, and foot, and are not directly involved in scrotal sensation relevant to an inguinal hernia repair.
- They also contribute to the **pudendal nerve**, which primarily supplies the perineum and external genitalia, but injury to this nerve is less common in routine inguinal hernia repair.
*S2-S4*
- These nerve roots primarily form the **pudendal nerve**, which innervates the **perineum** and external genitalia (including some scrotal sensation), but injury to these specific nerves is not a typical complication of routine inguinal hernia repair.
- They also contribute to the **pelvic splanchnic nerves**, controlling bladder and bowel function, which are unrelated to the described sensory deficit.
*L4-L5*
- These nerve roots primarily contribute to nerves supplying the **lower limb**, such as the **femoral nerve** and **sciatic nerve**, and do not directly innervate the scrotum.
- Injury to these roots would typically result in motor or sensory deficits of the **thigh and leg**, not isolated scrotal tingling.
*L2-L3*
- While L2 contributes to nerves supplying the inguinal region and scrotum (genitofemoral nerve), the **ilioinguinal nerve** originates from L1.
- The **lateral femoral cutaneous nerve**, which originates from L2-L3, innervates the **lateral thigh**, and its injury would cause tingling there, not in the scrotum.
Embryological basis of dermatomes US Medical PG Question 3: A 25-year-old woman comes to the physician because of pain and weakness in her right forearm and hand for several months. Two years ago, she sustained a fracture of her ulnar shaft with dislocation of the radial head that was treated surgically. Physical examination shows mild tenderness a few centimeters distal to the lateral epicondyle. She has marked weakness when attempting to extend her right middle finger. There is radial deviation on extension of the wrist. Sensation is not impaired. Which of the following nerves is most likely affected in this patient?
- A. Ulnar nerve
- B. Musculocutaneous nerve
- C. Posterior interosseous nerve (Correct Answer)
- D. Superficial radial nerve
- E. Anterior interosseous nerve
Embryological basis of dermatomes Explanation: ***Posterior interosseous nerve***
- The symptoms, including weakness in **middle finger extension** and **radial deviation of the wrist on extension**, are classic signs of **posterior interosseous nerve** (PIN) palsy. This nerve primarily innervates the muscles responsible for **finger and thumb extension**, as well as **extensor carpi ulnaris** (ECU) for wrist extension.
- The **radial deviation on wrist extension** occurs because the radial-sided wrist extensors (**extensor carpi radialis longus** and **brevis**) are innervated by the **radial nerve proper** before it branches into PIN, so they remain intact. With loss of ECU (ulnar-sided wrist extensor), unopposed action of ECRL and ECRB causes radial deviation.
- PIN palsy can result from **trauma** or compression, and the patient's history of a **radial head dislocation** two years prior is a significant risk factor for nerve damage in this region, particularly as PIN passes through the **supinator muscle** (arcade of Frohse). Tenderness distal to the **lateral epicondyle** also points to the region where PIN can be compressed.
*Ulnar nerve*
- An **ulnar nerve** injury would primarily cause weakness in **finger adduction and abduction** (especially the little finger and ring finger), **flexion of the ulnar half of the profundus**, and **intrinsic hand muscles**, leading to a "claw hand" deformity if severe.
- Sensation would also be affected in the **palmar and dorsal aspects of the 5th digit** and the **medial half of the 4th digit**, which is not described.
*Musculocutaneous nerve*
- The **musculocutaneous nerve** primarily innervates the **biceps brachii** and **brachialis muscles**, responsible for **elbow flexion** and **forearm supination**.
- Sensory deficits would be noted on the **lateral forearm**, none of which align with the patient's symptoms.
*Superficial radial nerve*
- The **superficial radial nerve** is purely sensory and provides sensation to the **dorsum of the hand** and parts of the thumb, index, and middle fingers.
- It does not have any motor function, so motor weakness would not be a symptom of its injury.
*Anterior interosseous nerve*
- The **anterior interosseous nerve** (AIN) is a purely motor branch of the median nerve, responsible for innervating the **flexor pollicis longus**, **flexor digitorum profundus (index and middle fingers)**, and **pronator quadratus**.
- Injury to the AIN would result in an inability to form an "OK" sign (due to impaired flexion of the thumb IP joint and index finger DIP joint) and no sensory loss.
Embryological basis of dermatomes US Medical PG Question 4: A group of investigators studying embryological defects in mice knock out a gene that is responsible for the development of the ventral wing of the third branchial pouch. A similar developmental anomaly in a human embryo is most likely to result in which of the following findings after birth?
- A. Cleft palate
- B. Discharging neck sinus (Correct Answer)
- C. Carpopedal spasm
- D. Conductive hearing loss
- E. White oral patches
Embryological basis of dermatomes Explanation: ***Discharging neck sinus***
- The **ventral wing of the third pharyngeal pouch** gives rise to the **thymus**
- During embryonic development, the thymus descends from the pharynx into the anterior mediastinum via the **thymopharyngeal duct**
- Normally, this duct obliterates completely, but **failure of obliteration** can result in a **cervical thymic cyst** or **persistent thymic tract**
- This presents as a **discharging neck sinus** along the lateral neck (anterior border of sternocleidomastoid), which may drain clear fluid or become infected
- This is a classic presentation of a **third pharyngeal pouch anomaly** affecting the thymic descent pathway
*Carpopedal spasm*
- **Carpopedal spasm** is a sign of **hypocalcemia** due to **hypoparathyroidism**
- The **dorsal wing** (not ventral wing) of the third pharyngeal pouch forms the **inferior parathyroid glands**
- Since the question specifically identifies a defect in the **ventral wing** (thymus), hypoparathyroidism would not result
- A dorsal wing defect would cause absent inferior parathyroid glands and hypocalcemia
*Cleft palate*
- Results from failure of **palatine shelf fusion** during weeks 8-12 of development
- Associated with **maxillary prominence** derivatives (first pharyngeal arch) and secondary palate formation
- Not related to third pharyngeal pouch development
*Conductive hearing loss*
- Associated with **first and second pharyngeal arch** derivatives affecting the middle ear structures
- First arch: malleus, incus (in part); Second arch: stapes (in part)
- The **third pharyngeal pouch** does not contribute to auditory structures
*White oral patches*
- Typically represent **mucosal lesions** (leukoplakia, candidiasis, lichen planus)
- Not associated with embryological defects of the pharyngeal apparatus
- Unrelated to third pharyngeal pouch derivatives
Embryological basis of dermatomes US Medical PG Question 5: 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
Embryological basis of dermatomes 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.
Embryological basis of dermatomes US Medical PG Question 6: A 59-year-old woman presents to the emergency room with severe low back pain. She reports pain radiating down her left leg into her left foot. She also reports intermittent severe lower back spasms. The pain started after lifting multiple heavy boxes at her work as a grocery store clerk. She denies bowel or bladder dysfunction. Her past medical history is notable for osteoporosis and endometrial cancer. She underwent a hysterectomy 20 years earlier. She takes alendronate. Her temperature is 99°F (37.2°C), blood pressure is 135/85 mmHg, pulse is 85/min, and respirations are 22/min. Her BMI is 21 kg/m^2. On exam, she is unable to bend over due to pain. Her movements are slowed to prevent exacerbating her muscle spasms. A straight leg raise elicits severe radiating pain into her left lower extremity. The patient reports that the pain is worst along the posterior thigh and posterolateral leg into the fourth and fifth toes. Palpation along the lumbar vertebral spines demonstrates mild tenderness. Patellar reflexes are 2+ bilaterally. The Achilles reflex is decreased on the left. Which nerve root is most likely affected in this patient?
- A. L5
- B. S2
- C. L3
- D. L4
- E. S1 (Correct Answer)
Embryological basis of dermatomes Explanation: ***S1***
- Pain radiating to the **posterior thigh**, **posterolateral leg**, and into the **fourth and fifth toes** is characteristic of **S1 dermatome involvement**.
- A **decreased Achilles reflex** (ankle jerk reflex) specifically points to compromise of the **S1 nerve root**.
*L5*
- **L5 radiculopathy** typically causes pain and sensory deficits in the **dorsum of the foot** and into the **first, second, and third toes**.
- Motor weakness often affects **foot dorsiflexion** and **toe extension**, not primarily the Achilles reflex.
*S2*
- **S2 radiculopathy** would primarily affect sensation along the **posterior thigh** and **calf**, with possible involvement of the **plantar aspect of the foot**.
- It does not typically cause a decrease in the **Achilles reflex**, which is predominantly S1.
*L3*
- **L3 radiculopathy** typically presents with pain and sensory changes along the **anterior thigh** and possibly the **medial knee**.
- It can affect the **patellar reflex**, which is intact in this patient, and does not cause pain in the posterior leg or foot.
*L4*
- **L4 radiculopathy** typically causes pain and sensory changes over the **anterior thigh**, **medial leg**, and potentially the **medial malleolus**.
- It often presents with weakness in **quadriceps muscle** and can cause a diminished **patellar reflex**, which is normal in this patient.
Embryological basis of dermatomes US Medical PG Question 7: 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
Embryological basis of dermatomes 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
Embryological basis of dermatomes US Medical PG Question 8: 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
Embryological basis of dermatomes 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.
Embryological basis of dermatomes US Medical PG Question 9: 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
Embryological basis of dermatomes 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.
Embryological basis of dermatomes US Medical PG Question 10: A 58-year-old diabetic man with multiple thoracic vertebral compression fractures presents with progressive lower extremity dysfunction. He has spastic paraparesis with hyperreflexia, bilateral Babinski signs, and a sensory level at T10. However, he also has areflexic bladder, saddle anesthesia, and absent bulbocavernosus reflex. Upper extremities are completely normal. Synthesize the anatomical explanation for this mixed upper and lower motor neuron presentation.
- A. Complete spinal cord transection at T10 level
- B. Conus medullaris syndrome with additional cord compression (Correct Answer)
- C. Cauda equina syndrome with coincidental cord pathology
- D. Combined central cord and conus medullaris injury
- E. Diabetic lumbosacral radiculoplexus neuropathy with cord compression
Embryological basis of dermatomes Explanation: ***Conus medullaris syndrome with additional cord compression***
- The **conus medullaris** represents the terminal end of the spinal cord (L1-L2), where damage causes a mix of **UMN signs** (from the cord itself) and **LMN signs** (from exiting sacral roots).
- This patient exhibits **UMN signs** (spasticity, hyperreflexia, Babinski) from cord compression at T10, alongside **LMN signs** (areflexic bladder, saddle anesthesia, absent bulbocavernosus reflex) specific to the **conus/sacral segments**.
*Complete spinal cord transection at T10 level*
- A complete transection at T10 would cause **pure UMN signs** below the level of the lesion and would not selectively cause isolated **areflexic bladder** or **saddle anesthesia** typical of conus/root injury.
- It would typically result in a **spastic bladder** over time once the period of spinal shock has resolved, unlike this patient's LMN presentation.
*Cauda equina syndrome with coincidental cord pathology*
- **Cauda equina syndrome** involves compression of lumbosacral nerve roots and results in a **pure LMN lesion** without Babinski signs or hyperreflexia.
- While cord pathology could explain UMN signs, the specific clustering of sacral symptoms with thoracic compression points more directly to the **conus medullaris** transition zone.
*Combined central cord and conus medullaris injury*
- **Central cord syndrome** almost exclusively affects the **cervical spine**, leading to upper extremity weakness greater than lower extremity weakness.
- Since the patient's **upper extremities** are normal, a central cord mechanism is anatomically inconsistent with this presentation.
*Diabetic lumbosacral radiculoplexus neuropathy with cord compression*
- **Diabetic radiculoplexus neuropathy** (amyotrophy) typically presents with **acute, asymmetric thigh pain** and proximal muscle wasting, which is not described here.
- While diabetes is a risk factor for neuropathies, it does not explain the anatomical clustering of **saddle anesthesia** and **absent bulbocavernosus reflex** as precisely as a conus lesion.
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