Cross-sectional anatomy — MCQs

A 24-day-old neonate is brought to the emergency department by his parents with high-grade fever, inability to feed, and lethargy. Since his birth, he was active and energetic, feeding every 2-3 hours and making 6-8 wet diapers every day until 2 days ago when he vomited twice, developed diarrhea, and slowly became lethargic. The boy was born at 39 weeks gestation via spontaneous vaginal delivery. He is up to date on all vaccines and is meeting all developmental milestones. He has not been in contact with any sick people lately. Today, his temperature is 39.4°C (102.9°F). He looks floppy and is unresponsive and difficult to rouse. Physical exam reveals a bulging anterior fontanelle. He is admitted to the NICU with the suspicion of neonatal meningitis, cerebrospinal fluid analysis is ordered, and empiric antibiotics are started. Which of the following structures will be punctured during the lumbar puncture procedure?

Q13

A 45-year-old man is brought to the emergency department by ambulance after a motor vehicle collision. He is not responsive to verbal commands and is unable to provide any history. His pulse is 108/min and regular. Physical examination shows ecchymoses over the neck and back. Neurological examination indicates damage to the spinal cord at the level shown in the illustration. This patient's injury is most likely located at which of the following levels of the spinal cord?

Q14

A 28-year-old man is admitted to the hospital for the evaluation of symmetric, ascending weakness that started in his feet and has become progressively worse over the past 5 days. A lumbar puncture is performed to confirm the diagnosis. As the needle is advanced during the procedure, there is resistance just before entering the epidural space. This resistance is most likely due to which of the following structures?

Cross-sectional anatomy US Medical PG Practice Questions and MCQs

Question 11: While performing a lumbar puncture on a patient presenting with fever, neck rigidity, and altered sensorium, what is the last structure to be encountered by the needle?

A. Dura mater
B. Pia mater
C. Ligamentum flavum
D. Arachnoid membrane (Correct Answer)
E. Supraspinous ligament

Explanation: ***Arachnoid membrane*** - The **arachnoid membrane** is the innermost layer the needle punctures before entering the **subarachnoid space** where cerebrospinal fluid (CSF) is collected. - The needle passes through skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, ligamentum flavum, epidural space, and dura mater, with arachnoid mater being the last barrier. *Dura mater* - The **dura mater** is pierced **before** the arachnoid membrane and before entering the subarachnoid space. - It is a tough, fibrous membrane forming the outermost layer of the **meninges**. *Pia mater* - The **pia mater** is in direct contact with the spinal cord and is **not punctured** during a properly performed lumbar puncture. - It is the innermost meningeal layer, adherent to the neural tissue itself. *Ligamentum flavum* - The **ligamentum flavum** is encountered **early** in the lumbar puncture procedure, prior to the epidural space and the meningeal layers. - It is a strong ligament connecting the laminae of adjacent vertebrae. *Supraspinous ligament* - The **supraspinous ligament** is one of the **first structures** encountered during lumbar puncture, after penetrating skin and subcutaneous tissue. - It connects the spinous processes of adjacent vertebrae and is pierced well before reaching the meningeal layers.

Question 12: A 24-day-old neonate is brought to the emergency department by his parents with high-grade fever, inability to feed, and lethargy. Since his birth, he was active and energetic, feeding every 2-3 hours and making 6-8 wet diapers every day until 2 days ago when he vomited twice, developed diarrhea, and slowly became lethargic. The boy was born at 39 weeks gestation via spontaneous vaginal delivery. He is up to date on all vaccines and is meeting all developmental milestones. He has not been in contact with any sick people lately. Today, his temperature is 39.4°C (102.9°F). He looks floppy and is unresponsive and difficult to rouse. Physical exam reveals a bulging anterior fontanelle. He is admitted to the NICU with the suspicion of neonatal meningitis, cerebrospinal fluid analysis is ordered, and empiric antibiotics are started. Which of the following structures will be punctured during the lumbar puncture procedure?

A. Nucleus pulposus
B. Denticulate ligament
C. Anterior Longitudinal Ligament
D. Dura layer (Correct Answer)
E. Pia layer

Explanation: ***Dura layer*** - During a **lumbar puncture**, the needle is inserted into the **subarachnoid space** to collect cerebrospinal fluid (CSF). To reach this space, the needle must pass through several layers, including the **dura mater** (and the closely adherent arachnoid mater). - The dura mater is the **outermost meningeal layer** and must be punctured to access the CSF that surrounds the nerve roots in the cauda equina. - The needle typically passes through: skin → subcutaneous tissue → supraspinous ligament → interspinous ligament → ligamentum flavum → epidural space → **dura mater** → subarachnoid space. *Nucleus pulposus* - The **nucleus pulposus** is the central, gelatinous core of the **intervertebral disc**, which is located anterior to the spinal canal. - Puncturing the nucleus pulposus would indicate a needle inserted too far anteriorly, potentially damaging the disc and not accessing the CSF. *Denticulate ligament* - The **denticulate ligaments** are extensions of the pia mater that anchor the spinal cord to the dura mater. - These ligaments are located at the level of the spinal cord (which ends at L1-L2), well above the typical lumbar puncture site (L3-L4 or L4-L5). They are not encountered during a properly performed lumbar puncture. *Anterior Longitudinal Ligament* - The **anterior longitudinal ligament** is a strong fibrous band that runs along the anterior surfaces of the vertebral bodies. - This ligament is far anterior to the spinal canal and the target location for a lumbar puncture; puncturing it would mean the needle passed entirely through the vertebral column. *Pia layer* - The **pia mater** is the innermost meningeal layer, which is intimately adhered to the surface of the spinal cord and individual nerve roots. - Lumbar puncture is performed **below the level of the spinal cord** (which terminates at L1-L2 as the conus medullaris) in the region of the **cauda equina**, where only nerve roots are present floating freely in CSF. - The pia mater covers individual nerve roots but is not a continuous layer at the LP site. The needle enters the subarachnoid space between nerve roots without puncturing the pia mater or the nerve roots themselves.

Question 13: A 45-year-old man is brought to the emergency department by ambulance after a motor vehicle collision. He is not responsive to verbal commands and is unable to provide any history. His pulse is 108/min and regular. Physical examination shows ecchymoses over the neck and back. Neurological examination indicates damage to the spinal cord at the level shown in the illustration. This patient's injury is most likely located at which of the following levels of the spinal cord?

A. C2
B. S2
C. L4
D. T6 (Correct Answer)
E. S4

Explanation: ***T6*** - The correct answer depends on identifying the spinal cord level shown in the illustration (cross-sectional anatomy). - Injury at **T6** (mid-thoracic level) can present with: - **Paraplegia** with loss of lower extremity function - **Sensory level** at approximately the xiphoid process (T6 dermatome) - Risk of **autonomic dysreflexia** if injury is complete (occurs with injuries at or above T6) - Preserved upper extremity function and respiratory capacity - The mid-thoracic spinal cord cross-section would show the characteristic butterfly-shaped gray matter with surrounding white matter tracts. *C2* - **C2** injury is a high cervical lesion that would result in: - **Quadriplegia** (tetraplegia) affecting all four extremities - Severe **respiratory compromise** requiring mechanical ventilation (phrenic nerve C3-C5) - High mortality if untreated - Cross-sectional anatomy at C2 shows enlarged gray matter for upper limb innervation. *S2* - **S2** injury is at the level of the conus medullaris/cauda equina and would cause: - **Bowel and bladder dysfunction** (loss of parasympathetic control) - **Saddle anesthesia** (loss of perineal sensation) - Possible lower extremity weakness - No significant autonomic instability - Would not explain the clinical presentation in the upper body. *L4* - **L4** injury affects the lumbar enlargement and would cause: - **Lower extremity weakness** (quadriceps, tibialis anterior) - **Diminished patellar reflex** - Sensory loss in the medial leg and foot - Bowel/bladder may be affected - Cross-section shows prominent gray matter for lower limb innervation. *S4* - **S4** is the lowest sacral level and injury would cause: - Very limited dysfunction: **anal sphincter weakness** - **Loss of anocutaneous reflex** - Minimal motor or sensory deficits elsewhere - Would not cause systemic neurological compromise.

Question 14: A 28-year-old man is admitted to the hospital for the evaluation of symmetric, ascending weakness that started in his feet and has become progressively worse over the past 5 days. A lumbar puncture is performed to confirm the diagnosis. As the needle is advanced during the procedure, there is resistance just before entering the epidural space. This resistance is most likely due to which of the following structures?

A. Dura mater
B. Supraspinous ligament
C. Interspinous ligament
D. Ligamentum flavum (Correct Answer)
E. Superficial fascia

Explanation: ***Ligamentum flavum*** - The **ligamentum flavum** is encountered after the interspinous ligament and is the last major ligament traversed before entering the **epidural space**. - It is a **thick, elastic, and strong ligament** that provides significant resistance to the needle, often described as a "pop" or "give" sensation as it is penetrated. *Dura mater* - The **dura mater** is the outermost membrane of the meninges and is encountered *after* the epidural space, surrounding the spinal cord. - While it provides some resistance, the more significant resistance *just before* the epidural space is due to the ligamentum flavum. *Supraspinous ligament* - The **supraspinous ligament** is a strong fibrous cord that connects the tips of the spinous processes. - It is located more superficially and is therefore encountered much earlier in the lumbar puncture procedure, not just before the epidural space. *Interspinous ligament* - The **interspinous ligament** connects adjacent spinous processes and is traversed *before* the ligamentum flavum. - It offers some resistance, but it is typically less prominent than the resistance felt when penetrating the thicker ligamentum flavum. *Superficial fascia* - The **superficial fascia** is located immediately beneath the skin and subcutaneous tissue. - It is encountered very early in the procedure and would not be the structure providing resistance just before the epidural space.

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