A 52-year-old man with multiple sclerosis presents with progressive bilateral leg weakness and spasticity over 3 years, now requiring a wheelchair. MRI shows demyelinating plaques in the cervical and thoracic spinal cord lateral columns bilaterally, corona radiata, and periventricular white matter. He has no sensory level but has impaired vibration sense in the feet. Upper extremities are minimally affected. Evaluate which anatomical principle best explains the leg-predominant motor disability despite multiple CNS lesions.
Q2
A 25-year-old woman presents after a motor vehicle accident with complete paralysis below T10. Initial examination shows flaccid paralysis, absent reflexes, and loss of all sensation below T10. Five weeks later, she develops spastic paralysis, hyperreflexia, clonus, and Babinski signs bilaterally below the lesion, but remains unable to move her legs voluntarily. Bladder function shows detrusor hyperreflexia. Evaluate the pathophysiological changes in the corticospinal system that explain this evolution.
Q3
A 42-year-old man with known cerebral arteriovenous malformation undergoes embolization. Post-procedure, he develops weakness of his right lower extremity (2/5) with preserved right upper extremity strength (5/5). He also has urinary incontinence and personality changes with apathy. MRI shows ischemic changes in the left medial frontal lobe. Evaluate the relationship between the ischemic location and the specific pattern of motor and associated deficits.
Q4
A 67-year-old man with uncontrolled hypertension presents with sudden right hemiplegia, right facial weakness, and leftward eye deviation. He is alert but has right-sided weakness (1/5 in arm and leg). CT shows a hemorrhage in the left corona radiata and internal capsule. Three days later, he develops increased weakness. Repeat CT shows hemorrhage expansion into the ventricles. Analyze the anatomical progression affecting the corticospinal system.
Q5
A 34-year-old woman presents with progressive weakness that began in her right hand and has spread to involve her right arm and leg over 18 months. She now has weakness in the left hand. Examination shows fasciculations, muscle atrophy, hyperreflexia, and both upper and lower motor neuron signs without sensory deficits. EMG shows widespread denervation. Analyze the anatomical levels of corticospinal tract involvement that best explain both upper and lower motor neuron signs.
Q6
A 58-year-old man develops sudden vertigo, dysphagia, and hoarseness. Examination shows right-sided ptosis, miosis, facial anhidrosis, loss of pain and temperature on the right face but left body, ataxia on the right, and palatal weakness on the right. He also has mild right-sided weakness of the extremities. Analyze which component of the corticospinal pathway is affected to explain the motor findings.
Q7
A 55-year-old alcoholic man presents with progressive difficulty walking. Examination reveals bilateral leg weakness (3/5 strength) with spasticity, hyperreflexia, and bilateral Babinski signs. Upper extremity strength is 5/5. Vibration and proprioception are severely impaired in the lower extremities. MRI shows degeneration of the dorsal and lateral columns of the spinal cord. Vitamin B12 levels are critically low. Apply your knowledge of corticospinal tract anatomy to explain the motor findings.
Q8
A 28-year-old man sustains a stab wound to the right side of his neck at the level of C5. On examination three months later, he has spastic paralysis of the right upper and lower extremities with hyperreflexia, but also has ipsilateral loss of fine touch and proprioception. Contralateral pain and temperature sensation is lost below the lesion. Which anatomical structure explains the ipsilateral motor deficits?
Q9
A 62-year-old woman with hypertension presents with sudden onset right-sided weakness that predominantly affects her right leg more than her right arm. She has 2/5 strength in the right lower extremity and 4/5 in the right upper extremity. There is hyperreflexia and an extensor plantar response on the right. Based on the pattern of weakness, which arterial territory is most likely affected?
Q10
A 45-year-old man presents to the emergency department with sudden onset of left-sided weakness. On examination, he has 2/5 strength in the left upper and lower extremities with hyperreflexia and a positive Babinski sign on the left. Sensation is intact bilaterally. MRI shows an acute infarct in the right cerebral hemisphere. Based on these findings, which specific anatomical structure is most likely affected?
Corticospinal tracts US Medical PG Practice Questions and MCQs
Question 1: A 52-year-old man with multiple sclerosis presents with progressive bilateral leg weakness and spasticity over 3 years, now requiring a wheelchair. MRI shows demyelinating plaques in the cervical and thoracic spinal cord lateral columns bilaterally, corona radiata, and periventricular white matter. He has no sensory level but has impaired vibration sense in the feet. Upper extremities are minimally affected. Evaluate which anatomical principle best explains the leg-predominant motor disability despite multiple CNS lesions.
A. Cervical cord lateral corticospinal tract plaques preferentially affect medially positioned leg fibers with longer distance from cortex
B. Corona radiata lesions selectively damage leg motor fibers due to vascular watershed vulnerability
C. Thoracic cord lesions interrupt leg motor function while cervical lesions spare arm innervation
D. Periventricular plaques selectively affect leg motor cortex projections
E. Demyelination affects longest corticospinal axons (to legs) more than shorter axons (to arms) due to length-dependent vulnerability (Correct Answer)
Explanation: ***Demyelination affects longest corticospinal axons (to legs) more than shorter axons (to arms) due to length-dependent vulnerability***
- The **corticospinal tract** axons innervating the lower limbs are the longest in the body, traveling from the **motor cortex** all the way to the lumbar/sacral spinal cord.
- Longer axons have a greater **total myelin surface area** and higher metabolic demands, making them more susceptible to cumulative damage from **demyelinating plaques** across multiple CNS levels.
*Cervical cord lateral corticospinal tract plaques preferentially affect medially positioned leg fibers with longer distance from cortex*
- While it is true that leg fibers are found more **medially** in the lateral corticospinal tract compared to arm fibers, distance from the cortex is not the primary anatomical driver in this localized context.
- MS plaques are typically **multifocal** and do not consistently show a preference for the medial aspect of the tract based solely on fiber distance.
*Corona radiata lesions selectively damage leg motor fibers due to vascular watershed vulnerability*
- **Multiple sclerosis** is an inflammatory, immune-mediated disease of **white matter**, not a primary vascular or ischemic stroke-related disorder.
- **Watershed vulnerability** explains specific types of infarctions but does not account for the pattern of progressive demyelination seen in MS.
*Thoracic cord lesions interrupt leg motor function while cervical lesions spare arm innervation*
- This patient has documented lesions in the **cervical spinal cord**, which should theoretically affect both upper and lower limb fibers if localized injury were the only factor.
- This explanation fails to account for why the **cervical plaques** (which contain fibers for both arms and legs) preferentially result in **leg-predominant** deficits.
*Periventricular plaques selectively affect leg motor cortex projections*
- **Periventricular plaques** are common in MS but they do not specifically target fibers for the legs more than those for other body parts based on cortical mapping alone.
- The disability pattern in MS is better explained by the **cumulative effect** of multiple lesions on the longest axonal pathways rather than a specific cortical localization.
Question 2: A 25-year-old woman presents after a motor vehicle accident with complete paralysis below T10. Initial examination shows flaccid paralysis, absent reflexes, and loss of all sensation below T10. Five weeks later, she develops spastic paralysis, hyperreflexia, clonus, and Babinski signs bilaterally below the lesion, but remains unable to move her legs voluntarily. Bladder function shows detrusor hyperreflexia. Evaluate the pathophysiological changes in the corticospinal system that explain this evolution.
A. Corticospinal tract regeneration with aberrant connections
B. Spinal shock resolution unmasking intrinsic spinal cord reflex activity below the level of complete corticospinal tract disruption (Correct Answer)
C. Development of secondary syrinx with incomplete corticospinal tract injury
D. Wallerian degeneration completion with partial reinnervation
E. Sprouting of corticospinal axons creating new but non-functional pathways
Explanation: ***Spinal shock resolution unmasking intrinsic spinal cord reflex activity below the level of complete corticospinal tract disruption***
- The transition from **flaccid paralysis** and areflexia to **spastic paralysis** and hyperreflexia characterizes the resolution of **spinal shock** after an acute spinal cord injury.
- When the **corticospinal tract** is disrupted, the loss of descending inhibition allows **spinal reflex arcs** to become hyperactive, leading to clonus, Babinski signs, and **detrusor hyperreflexia**.
*Corticospinal tract regeneration with aberrant connections*
- In the central nervous system, **axonal regeneration** is minimal and insufficient to explain the systematic return of reflexes or the lack of voluntary movement.
- The observed symptoms are due to **denervation hypersensitivity** and synaptic changes in the distal cord, not the regrowth of long-tract fibers.
*Development of secondary syrinx with incomplete corticospinal tract injury*
- A **syringomyelia** (syrinx) typically presents months to years later with a **dissociated sensory loss** and worsening function, rather than the standard evolution of spasticity.
- The scenario describes a **complete lesion** evolution (loss of all sensation and movement), whereas a syrinx would imply new, often progressive, segmental deficits.
*Wallerian degeneration completion with partial reinnervation*
- While **Wallerian degeneration** occurs distal to the injury site, it leads to the permanent loss of the descending motor pathway rather than reinnervation.
- Partial reinnervation would typically manifest as some return of **voluntary motor control**, which is explicitly absent in this patient.
*Sprouting of corticospinal axons creating new but non-functional pathways*
- While some local **interneuronal sprouting** occurs within the spinal cord, it does not involve the long-distance descending **corticospinal axons** across the injury site.
- The development of **hyperreflexia** is driven by the reorganization of local spinal circuits below the lesion, not by non-functional corticospinal sprouts.
Question 3: A 42-year-old man with known cerebral arteriovenous malformation undergoes embolization. Post-procedure, he develops weakness of his right lower extremity (2/5) with preserved right upper extremity strength (5/5). He also has urinary incontinence and personality changes with apathy. MRI shows ischemic changes in the left medial frontal lobe. Evaluate the relationship between the ischemic location and the specific pattern of motor and associated deficits.
A. Anterior cerebral artery territory infarct affecting leg area of motor cortex and prefrontal regions (Correct Answer)
B. Middle cerebral artery watershed infarct with selective vulnerability
C. Paracentral lobule infarct sparing supplementary motor area
D. Corona radiata infarct with selective leg fiber involvement
E. Internal capsule lacunar infarct with behavioral manifestations
Explanation: ***Anterior cerebral artery territory infarct affecting leg area of motor cortex and prefrontal regions***
- The **anterior cerebral artery (ACA)** supplies the **medial aspect** of the cerebral hemisphere; a stroke here characteristically causes **contralateral leg weakness** while sparing the face and arm.
- Involvement of the **prefrontal cortex** and **medial frontal lobe** accounts for the **personality changes (apathy)**, and damage to the **micturition center** leads to **urinary incontinence**.
*Middle cerebral artery watershed infarct with selective vulnerability*
- **Watershed infarcts** typically occur between major territories (e.g., ACA-MCA) and present with proximal muscle weakness, often termed "**man-in-a-barrel**" syndrome.
- This pattern usually involves **bilateral** symptoms or affects the upper extremity more than the leg, which contradicts the focal **medial frontal** MRI finding.
*Paracentral lobule infarct sparing supplementary motor area*
- While a **paracentral lobule** lesion explains the **leg weakness** and **urinary incontinence**, it would not typically account for the **personality changes**.
- Apathy and executive dysfunction require broader involvement of the **prefrontal sections** of the ACA territory, rather than a localized posterior medial lesion.
*Corona radiata infarct with selective leg fiber involvement*
- A lesion in the **corona radiata** or **posterior limb of the internal capsule** could cause motor deficits, but these fibers are usually tightly packed, often causing **hemiparesis** (arm and leg).
- Subcortical lesions in these white matter tracts do not explain cortical signs like **behavioral changes** or **apathy**.
*Internal capsule lacunar infarct with behavioral manifestations*
- **Lacunar infarcts** in the internal capsule primarily present as **pure motor stroke**, affecting the face, arm, and leg relatively equally.
- They lack **cortical signs** such as the **apathy** and **personality changes** seen with frontal lobe cortical ischemia.
Question 4: A 67-year-old man with uncontrolled hypertension presents with sudden right hemiplegia, right facial weakness, and leftward eye deviation. He is alert but has right-sided weakness (1/5 in arm and leg). CT shows a hemorrhage in the left corona radiata and internal capsule. Three days later, he develops increased weakness. Repeat CT shows hemorrhage expansion into the ventricles. Analyze the anatomical progression affecting the corticospinal system.
A. Extension of hemorrhage destroyed remaining corticospinal fibers in internal capsule and disrupted cortical input (Correct Answer)
B. Development of hydrocephalus causing bilateral corticospinal tract compression
C. Intraventricular blood caused brainstem compression at the peduncles
D. Vasospasm reduced perfusion to the contralateral motor cortex
E. Hemorrhage extended to the medullary pyramids
Explanation: ***Extension of hemorrhage destroyed remaining corticospinal fibers in internal capsule and disrupted cortical input***
- The **internal capsule**, specifically the **posterior limb**, contains densely packed **corticospinal fibers**; expansion of a hemorrhage in this region destroys the remaining functional motor pathways.
- Progression into the **ventricles** often involves tracking through the **corona radiata**, further severing the white matter tracts that carry descending motor commands from the **primary motor cortex**.
*Development of hydrocephalus causing bilateral corticospinal tract compression*
- While **intraventricular hemorrhage** can lead to **obstructive hydrocephalus**, this typically presents with global neurological decline and signs of **increased intracranial pressure**, rather than worsening focal hemiplegia.
- Compression from hydrocephalus would likely cause **bilateral deficits** or affect the paracentral lobule fibers first, rather than worsening existing unilateral weakness to 0/5.
*Intraventricular blood caused brainstem compression at the peduncles*
- **Brainstem compression** at the level of the **cerebral peduncles** usually results from **transtentorial herniation**, which would present with a decreased level of consciousness and pupillary changes.
- Isolated focal worsening of hemiplegia is more characteristic of local lesion expansion in the **capsular region** rather than distant mass effect on the midbrain.
*Vasospasm reduced perfusion to the contralateral motor cortex*
- **Vasospasm** is a characteristic complication specifically of **subarachnoid hemorrhage**, typically occurring 3 to 14 days after the initial bleed.
- In **intraparenchymal hypertensive hemorrhage**, neurological worsening is more commonly due to **hematoma expansion** or peri-focal edema rather than arterial vasospasm.
*Hemorrhage extended to the medullary pyramids*
- The **medullary pyramids** are located in the lower brainstem; a hemorrhage originating in the **internal capsule** and corona radiata does not mechanically
Question 5: A 34-year-old woman presents with progressive weakness that began in her right hand and has spread to involve her right arm and leg over 18 months. She now has weakness in the left hand. Examination shows fasciculations, muscle atrophy, hyperreflexia, and both upper and lower motor neuron signs without sensory deficits. EMG shows widespread denervation. Analyze the anatomical levels of corticospinal tract involvement that best explain both upper and lower motor neuron signs.
A. Motor cortex and anterior horn cells (Correct Answer)
B. Internal capsule and peripheral nerves
C. Medullary pyramids only
D. Lateral corticospinal tracts bilaterally in spinal cord
E. Cerebral peduncles and ventral roots
Explanation: ***Motor cortex and anterior horn cells***
- The patient presents with **Amyotrophic Lateral Sclerosis (ALS)**, which is characterized by the simultaneous degeneration of **Upper Motor Neurons (UMN)** and **Lower Motor Neurons (LMN)**.
- Degeneration of the **motor cortex** (Betz cells) leads to UMN signs like **hyperreflexia**, while destruction of the **anterior horn cells** in the spinal cord leads to LMN signs like **fasciculations** and **atrophy**.
*Internal capsule and peripheral nerves*
- Lesions in the **internal capsule** would cause pure **UMN signs** (spasticity, hyperreflexia) without the widespread muscle atrophy seen here.
- **Peripheral nerve** involvement typically presents with **sensory deficits**, which are notably absent in this clinical scenario.
*Medullary pyramids only*
- Damage to the **medullary pyramids** would result in strictly **UMN signs** below the level of the lesion because LMNs are not located there.
- This isolated anatomical location cannot explain the **fasciculations** or the progressive involvement of multiple limbs across different spinal segments.
*Lateral corticospinal tracts bilaterally in spinal cord*
- While damage here explains **hyperreflexia** and spasticity, it does not account for the **denervation** and **fasciculations** seen on EMG.
- LMN signs require involvement of the **gray matter** (anterior horns) or the nerves exiting them, not just the **white matter tracts**.
*Cerebral peduncles and ventral roots*
- The **cerebral peduncles** contain descending UMN fibers, but a lesion here would not produce the widespread, multi-level **LMN signs** observed in the hands.
- While **ventral root** damage causes LMN signs, the clinical progression and lack of sensory loss are most consistent with a primary **neuronal cell body** disease (ALS).
Question 6: A 58-year-old man develops sudden vertigo, dysphagia, and hoarseness. Examination shows right-sided ptosis, miosis, facial anhidrosis, loss of pain and temperature on the right face but left body, ataxia on the right, and palatal weakness on the right. He also has mild right-sided weakness of the extremities. Analyze which component of the corticospinal pathway is affected to explain the motor findings.
A. Right cerebral peduncle involvement affecting pre-decussation fibers (Correct Answer)
B. Right medullary pyramid with partial involvement
C. Left internal capsule with crossed findings
D. Bilateral cortical involvement
E. Right lateral corticospinal tract in cervical cord
Explanation: ***Right cerebral peduncle involvement affecting pre-decussation fibers***
- The patient exhibits features of **lateral medullary (Wallenberg) syndrome**, but the addition of **ipsilateral (right-sided) hemiparesis** indicates involvement of the **corticospinal tract** before its decussation.
- Damage to the **right cerebral peduncle** or upper brainstem regions before the decussation in the lower medulla results in weakness on the same side relative to the cranial nerve deficits.
*Right medullary pyramid with partial involvement*
- The **medullary pyramid** is located medially; damage here usually causes **contralateral hemiparesis** as the fibers have not yet crossed the midline at this level.
- This location is typically spared in **Wallenberg syndrome**, which primarily affects the dorsolateral (retro-olivary) region of the medulla.
*Left internal capsule with crossed findings*
- A lesion in the **left internal capsule** would result in **right-sided weakness**, but it would not explain the right-sided **Horner's syndrome** or the crossed sensory loss pattern.
- The constellation of **dysphagia**, **hoarseness**, and cerebellar ataxia is specific to a brainstem localization, not the hemispheric white matter.
*Bilateral cortical involvement*
- **Bilateral cortical involvement** would typically present with more global cognitive deficits, seizures, or symmetric motor loss, which is not seen here.
- It does not account for the **crossed sensory loss** (ipsilateral face, contralateral body) which is a hallmark of a **dorsolateral medullary lesion**.
*Right lateral corticospinal tract in cervical cord*
- While a lesion in the **cervical cord** causes ipsilateral weakness, it does not explain **cranial nerve IX and X palsies** (dysphagia, hoarseness, palatal weakness).
- Cervical cord lesions would not cause the **trigeminal sensory loss** on the face or the prominent **vertigo** associated with the vestibular nuclei in the medulla.
Question 7: A 55-year-old alcoholic man presents with progressive difficulty walking. Examination reveals bilateral leg weakness (3/5 strength) with spasticity, hyperreflexia, and bilateral Babinski signs. Upper extremity strength is 5/5. Vibration and proprioception are severely impaired in the lower extremities. MRI shows degeneration of the dorsal and lateral columns of the spinal cord. Vitamin B12 levels are critically low. Apply your knowledge of corticospinal tract anatomy to explain the motor findings.
A. Bilateral anterior corticospinal tract degeneration
B. Bilateral lateral corticospinal tract degeneration in the spinal cord (Correct Answer)
C. Bilateral cortical motor neuron degeneration
D. Degeneration at the medullary pyramidal decussation
E. Bilateral ventral horn cell degeneration
Explanation: ***Bilateral lateral corticospinal tract degeneration in the spinal cord***
- **Subacute combined degeneration (SCD)** of the spinal cord involves the **lateral corticospinal tracts**, leading to upper motor neuron signs such as **spasticity**, **hyperreflexia**, and **bilateral Babinski signs**.
- The anatomical localization in the **spinal cord** explains why the upper extremities may be spared or less affected compared to the lower extremities depending on the level of degeneration.
*Bilateral anterior corticospinal tract degeneration*
- The **anterior corticospinal tract** primarily controls the **axial and proximal musculature**, whereas this patient presents with significant limb weakness.
- **SCD** predominantly affects the **lateral** tracts, and anterior tract involvement alone would not explain the classic presentation of **spastic paraplegia**.
*Bilateral cortical motor neuron degeneration*
- Degeneration of the **motor cortex** (upper motor neurons) would typically involve widespread symptoms and is characteristic of diseases like **Amyotrophic Lateral Sclerosis (ALS)**.
- This patient's symptoms are localized to the **spinal cord** tracts, and the sensory findings (vibration/proprioception loss) specifically point to **dorsal column** involvement alongside the motor tracts.
*Degeneration at the medullary pyramidal decussation*
- A lesion at the **medullary pyramids** would cause dense **quadriparesis** rather than sparing the upper extremities.
- **Vitamin B12 deficiency** characteristically targets the white matter within the **spinal cord segments** rather than the brainstem nuclei or decussations.
*Bilateral ventral horn cell degeneration*
- **Ventral horn cell** damage results in **lower motor neuron (LMN)** signs such as flaccidity, atrophy, and **hyporeflexia**.
- This patient exhibits **upper motor neuron (UMN)** signs like **spasticity** and **hyperreflexia**, which are inconsistent with ventral horn (grey matter) destruction.
Question 8: A 28-year-old man sustains a stab wound to the right side of his neck at the level of C5. On examination three months later, he has spastic paralysis of the right upper and lower extremities with hyperreflexia, but also has ipsilateral loss of fine touch and proprioception. Contralateral pain and temperature sensation is lost below the lesion. Which anatomical structure explains the ipsilateral motor deficits?
A. Lateral corticospinal tract before decussation
B. Lateral corticospinal tract after decussation at the medulla (Correct Answer)
C. Anterior corticospinal tract
D. Corticobulbar tract
E. Rubrospinal tract
Explanation: ***Lateral corticospinal tract after decussation at the medulla***
- In **Brown-Séquard syndrome**, a spinal cord hemisection at C5 results in **ipsilateral spastic paralysis** because the lateral corticospinal tract decussates at the **medullary pyramids** before descending.
- Damage to the tract below the point of decussation leads to motor loss on the **same side** as the lesion, accompanied by hyperreflexia and spasticity.
*Lateral corticospinal tract before decussation*
- A lesion to the tract before it crosses the midline (above the medulla) would result in **contralateral** motor deficits, not ipsilateral.
- The **medullary decussation** is the anatomical boundary that determines whether a motor lesion presents on the same or opposite side of the body.
*Anterior corticospinal tract*
- This tract primarily controls **proximal/axial muscles** and does not decussate in the medulla, but rather at the level of the spinal segment it innervates.
- It accounts for only about 10-15% of corticospinal fibers and is not responsible for the **spastic paralysis** of the extremities seen in this case.
*Corticobulbar tract*
- The **corticobulbar tract** carries motor information to the **cranial nerve nuclei** in the brainstem, not down to the spinal cord level C5.
- Lesions here would present with deficits in facial movement, chewing, or swallowing, rather than **extremity paralysis**.
*Rubrospinal tract*
- This tract is an **extrapyramidal** motor pathway involved in mediating voluntary movement, primarily affecting the flexor muscle tone.
- While it resides in the lateral funiculus, the **lateral corticospinal tract** is the primary pathway whose disruption causes the classic Upper Motor Neuron signs described.
Question 9: A 62-year-old woman with hypertension presents with sudden onset right-sided weakness that predominantly affects her right leg more than her right arm. She has 2/5 strength in the right lower extremity and 4/5 in the right upper extremity. There is hyperreflexia and an extensor plantar response on the right. Based on the pattern of weakness, which arterial territory is most likely affected?
A. Right middle cerebral artery
B. Left anterior cerebral artery (Correct Answer)
C. Left middle cerebral artery
D. Right posterior cerebral artery
E. Basilar artery
Explanation: ***Left anterior cerebral artery***
- The **anterior cerebral artery (ACA)** supplies the medial aspect of the cerebral cortex, where the **motor homunculus** area for the leg and foot is located.
- A **contralateral** stroke pattern where the **lower extremity** is more severely affected than the upper extremity is the hallmark of an ACA territory infarct.
*Right middle cerebral artery*
- An infarct in the right MCA would cause **left-sided** weakness, which contradicts the right-sided symptoms found in this patient.
- MCA strokes typically involve the **face and arm** more than the leg due to the lateral cortical distribution of the vessel.
*Left middle cerebral artery*
- While it causes right-sided weakness, the **MCA territory** typically presents with **arm and face** deficits greater than leg deficits.
- Global involvement of the MCA may cause **aphasia** if the dominant hemisphere is involved, but the leg is relatively spared compared to the ACA.
*Right posterior cerebral artery*
- PCA strokes primarily present with **visual field defects** like homonymous hemianopia with macular sparing, rather than primary motor weakness.
- Motor deficits in PCA territory are rare unless the **midbrain** or **thalamus** (via deep branches) is involved, and would still be on the left side.
*Basilar artery*
- Basilar artery occlusion usually presents with **bilateral** motor clusters, cranial nerve palsies, or **impaired consciousness** (locked-in syndrome).
- It affects the **brainstem**, often resulting in
Question 10: A 45-year-old man presents to the emergency department with sudden onset of left-sided weakness. On examination, he has 2/5 strength in the left upper and lower extremities with hyperreflexia and a positive Babinski sign on the left. Sensation is intact bilaterally. MRI shows an acute infarct in the right cerebral hemisphere. Based on these findings, which specific anatomical structure is most likely affected?
A. Right lateral corticospinal tract in the spinal cord
B. Right internal capsule posterior limb (Correct Answer)
C. Right medullary pyramid
D. Right cerebral peduncle
E. Right precentral gyrus only
Explanation: ***Right internal capsule posterior limb***
- The **posterior limb of the internal capsule** contains densely packed fibers of the **corticospinal tract**, making it a common site for lacunar infarcts leading to **pure motor hemiparesis**.
- A lesion here results in **contralateral motor deficit** affecting both the upper and lower extremities equally without sensory loss, consistent with the patient's presentation.
*Right lateral corticospinal tract in the spinal cord*
- A lesion in the lateral corticospinal tract in the spinal cord would cause **ipsilateral** (same-side) motor deficits, not contralateral weakness.
- This location is excluded by the MRI findings confirming an **acute infarct in the right cerebral hemisphere**.
*Right medullary pyramid*
- While the pyramids contain descending motor fibers, they are located in the **brainstem**, not the cerebral hemisphere as indicated by the MRI.
- Lesions at this level typically present with different clinical syndromes, such as **Medial Medullary Syndrome**, often involving cranial nerve deficits.
*Right cerebral peduncle*
- The cerebral peduncles are located in the **midbrain** and contain descending motor fibers; however, they are not situated in the cerebral hemisphere proper.
- A lesion here would often be associated with other midbrain signs, such as an **ipsilateral oculomotor (CN III) nerve palsy**.
*Right precentral gyrus only*
- An isolated lesion of the **precentral gyrus** (primary motor cortex) usually follows a **somatotopic distribution**, making it less likely to affect the upper and lower limbs equally without extensive cortical damage.
- Pure motor weakness involving the entire contralateral side suggests involving the more compact fibers in the **internal capsule** rather than just the gyrus.
