USMLE Step 1 Quiz: Neuroanatomy And Localization

What this quiz covers

This quiz focuses on Neuroanatomy And Localization, giving you a quick way to practice the rules, question types, and explanations that matter most for USMLE Step 1.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Left superior temporal gyrus (Wernicke area) (correct answer)
2. Left inferior frontal gyrus (Broca area)
3. Right angular gyrus
4. Left primary motor cortex (leg area)
5. Right thalamus

Explanation: This question tests neuroanatomy and localization skills related to the nervous system. Understanding neuroanatomy involves recognizing how specific structures correlate with clinical symptoms. In this vignette, the acute confusion and fluent aphasia with left posterior temporal infarct helps identify the affected region. The correct answer, Left superior temporal gyrus (Wernicke area), accurately corresponds to the described symptoms and findings. A common incorrect choice, Left inferior frontal gyrus (Broca area), fails because it misinterprets the fluent nature as non-fluent aphasia. To improve skills, focus on correlating clinical signs with anatomical locations and practice interpreting imaging studies accurately. Differentiating aphasia types aids localization.

Question 2

1. Brown-Séquard syndrome (correct answer)
2. Anterior cord syndrome
3. Central cord syndrome
4. Cauda equina syndrome
5. Posterior cord syndrome

Explanation: This question tests neuroanatomy and localization skills related to the nervous system. Understanding neuroanatomy involves recognizing how specific structures correlate with clinical symptoms. In this vignette, the right-sided weakness and vibration loss below C5 with right C5 lesion helps identify the affected region. The correct answer, Brown-Séquard syndrome, accurately corresponds to the described symptoms and findings. A common incorrect choice, Central cord syndrome, fails because it misinterprets the ipsilateral pattern as bilateral. To improve skills, focus on correlating clinical signs with anatomical locations and practice interpreting imaging studies accurately. Recognizing hemicord patterns is crucial.

Question 3

1. Radial nerve (correct answer)
2. Median nerve
3. Ulnar nerve
4. Axillary nerve
5. Musculocutaneous nerve

Explanation: This question tests neuroanatomy and localization skills related to the nervous system. Understanding neuroanatomy involves recognizing how specific structures correlate with clinical symptoms. In this vignette, the wrist drop and dorsal hand numbness after humeral shaft fracture helps identify the affected region. The correct answer, Radial nerve, accurately corresponds to the described symptoms and findings. A common incorrect choice, Median nerve, fails because it misinterprets wrist drop as finger flexion deficit. To improve skills, focus on correlating clinical signs with anatomical locations and practice interpreting imaging studies accurately. Knowing nerve courses around bones is vital.

Question 4

1. CN II
2. CN III (correct answer)
3. CN IV
4. CN VI
5. CN VII

Explanation: This question tests neuroanatomy and localization skills related to the nervous system. Understanding neuroanatomy involves recognizing how specific structures correlate with clinical symptoms. In this vignette, the ptosis, mydriasis, and down-and-out right eye after PCOM aneurysm help identify the affected region. The correct answer, CN III, accurately corresponds to the described symptoms and findings. A common incorrect choice, CN IV, fails because it misinterprets the pupillary involvement. To improve skills, focus on correlating clinical signs with anatomical locations and practice interpreting imaging studies accurately. Knowing aneurysm compression effects is crucial.

Question 5

1. Medial medulla (pyramids)
2. Lateral medulla (nucleus ambiguus region) (correct answer)
3. Midbrain (oculomotor nucleus)
4. Ventral pons (basis pontis)
5. Cerebral peduncle (crus cerebri)

Explanation: This question tests neuroanatomy and localization skills related to the nervous system. Understanding neuroanatomy involves recognizing how specific structures correlate with clinical symptoms. In this vignette, the sudden vertigo, dysphagia, hoarseness, and crossed pain loss with PICA infarct help identify the affected region. The correct answer, Lateral medulla (nucleus ambiguus region), accurately corresponds to the described symptoms and findings. A common incorrect choice, Medial medulla (pyramids), fails because it misinterprets the sensory and cranial nerve findings as purely motor. To improve skills, focus on correlating clinical signs with anatomical locations and practice interpreting imaging studies accurately. Recognizing brainstem vascular syndromes like Wallenberg is essential.

Question 6

1. CN III
2. CN IV
3. CN VI (correct answer)
4. CN VII
5. CN V

Explanation: This question tests neuroanatomy and localization skills related to the nervous system. Understanding neuroanatomy involves recognizing how specific structures correlate with clinical symptoms. In this vignette, the sudden binocular diplopia and inability to abduct right eye with pontine lesion help identify the affected region. The correct answer, CN VI, accurately corresponds to the described symptoms and findings. A common incorrect choice, CN III, fails because it misinterprets abduction deficit as adduction. To improve skills, focus on correlating clinical signs with anatomical locations and practice interpreting imaging studies accurately. Reviewing extraocular muscle innervation is beneficial.

Question 7

1. CN V
2. CN VII (correct answer)
3. CN IX
4. CN X
5. CN XII

Explanation: This question tests neuroanatomy and localization skills related to the nervous system. Understanding neuroanatomy involves recognizing how specific structures correlate with clinical symptoms. In this vignette, the right facial paralysis including forehead and loss of taste with facial canal enhancement helps identify the affected region. The correct answer, CN VII, accurately corresponds to the described symptoms and findings. A common incorrect choice, CN V, fails because it misinterprets motor and taste deficits as sensory. To improve skills, focus on correlating clinical signs with anatomical locations and practice interpreting imaging studies accurately. Differentiating facial nerve functions is essential.

Question 8

1. Right anterior inferior cerebellar artery supplying the lateral pons
2. Right posterior inferior cerebellar artery supplying the lateral medulla (correct answer)
3. Right paramedian branches of the basilar artery supplying the medial pons
4. Right anterior spinal artery supplying the medial medulla

Explanation: When you encounter a patient with crossed neurological signs (deficits on opposite sides of the body for different sensory modalities), think brainstem stroke. The key is identifying which specific brainstem syndrome matches the clinical presentation. This patient presents with classic Wallenberg syndrome (lateral medullary syndrome). The constellation of ipsilateral facial pain/temperature loss, contralateral body pain/temperature loss, hoarseness, dysphagia, nystagmus, and ipsilateral ataxia points to a lateral medullary infarction. The posterior inferior cerebellar artery (PICA) supplies this region, affecting the spinal trigeminal tract (ipsilateral facial pain/temperature), spinothalamic tract (contralateral body pain/temperature), nucleus ambiguus (hoarseness and dysphagia), vestibular nuclei (nystagmus), and inferior cerebellar peduncle (ataxia). Choice A is incorrect because anterior inferior cerebellar artery (AICA) infarction affects the lateral pons, typically causing hearing loss and facial paralysis, not the medullary symptoms described. Choice C is wrong because medial pontine infarction would cause hemiparesis and internuclear ophthalmoplegia, not the crossed sensory findings seen here. Choice D is incorrect because anterior spinal artery infarction causes medial medullary syndrome with alternating hypoglossal hemiplegia—tongue deviation and contralateral weakness—not the lateral medullary findings present. Remember the "4 D's" of Wallenberg syndrome: Dysphagia, Dysphonia, Dizziness, and ipsilateral facial/contralateral body sensory Dissociation. This classic pattern immediately points to PICA territory infarction in the lateral medulla.

Question 9

1. Right frontal eye field in the cerebral cortex
2. Left frontal eye field in the cerebral cortex (correct answer)
3. Right paramedian pontine reticular formation
4. Left abducens nucleus in the dorsal pons

Explanation: When you encounter conjugate gaze deviation, you need to understand the neural control of horizontal eye movements. The frontal eye fields (FEF) in the cerebral cortex initiate voluntary horizontal gaze toward the contralateral side, while the paramedian pontine reticular formation (PPRF) coordinates the actual eye movements. This patient shows conjugate deviation toward the right with preserved vestibulo-ocular reflex (VOR). The intact VOR indicates that the brainstem gaze centers (PPRF and abducens nucleus) are functioning normally, since the VOR bypasses cortical input and directly tests brainstem pathways. Since the eyes deviate toward the right, the lesion must involve the structure responsible for leftward gaze. The left frontal eye field normally drives leftward gaze. When damaged, the unopposed right FEF causes the eyes to deviate toward the ipsilateral (right) side of the intact cortex. This explains both the rightward deviation and why it's a cortical rather than brainstem lesion. Choice A (right frontal eye field) would cause leftward deviation, not rightward. Choice C (right PPRF) would also cause leftward deviation and would affect the VOR, which is preserved here. Choice D (left abducens nucleus) would cause isolated left eye abduction weakness rather than conjugate deviation, and would also impair the VOR. Remember this key principle: cortical gaze lesions cause deviation toward the side of the lesion (eyes look toward the stroke), while brainstem lesions cause deviation away from the lesion. Always check if VOR is preserved to distinguish cortical from brainstem pathology.

Question 10

1. Lateral geniculate nucleus of the thalamus bilaterally
2. Optic tracts just distal to the optic chiasm on both sides
3. Optic chiasm at the midline where nasal fibers decussate (correct answer)
4. Meyer's loops within the temporal lobes bilaterally

Explanation: When you encounter visual field defects on the USMLE, systematically map the anatomy from retina to visual cortex to localize the lesion. The key is understanding which fibers cross at the optic chiasm and how different lesion locations create characteristic patterns. This patient's bilateral peripheral vision loss with central sparing indicates a bitemporal hemianopia. At the optic chiasm, nasal retinal fibers (which detect temporal visual fields) decussate to join the contralateral optic tract. A lesion at this crossing point selectively damages these crossing fibers while sparing uncrossed temporal retinal fibers that detect nasal/central fields. This creates the classic "tunnel vision" pattern seen here, often caused by pituitary adenomas compressing the chiasm from below. Choice A (lateral geniculate nucleus bilaterally) would cause complete visual field defects or complex patterns, not the specific bitemporal pattern described. Choice B (bilateral optic tracts) would produce bilateral homonymous hemianopias—each tract carries fibers from both eyes representing the same visual field, so damage causes loss of the same side of vision in both eyes. Choice D (Meyer's loops bilaterally) would cause bilateral superior quadrantanopias ("pie in the sky" defects), as these temporal lobe fibers carry information from inferior retinal quadrants. Remember: bitemporal hemianopia = chiasmal lesion. The chiasm is the only location where a single midline lesion can selectively affect temporal fields from both eyes. Think "pituitary tumor" when you see this pattern, especially with the normal funduscopic exam ruling out retinal pathology.

Question 11

1. Right nucleus ambiguus in the medulla (correct answer)
2. Left corticobulbar fibers to nucleus ambiguus
3. Right hypoglossal nucleus in the medulla
4. Left solitary nucleus in the medulla

Explanation: When you encounter a patient with speech changes and swallowing difficulties after stroke, think about cranial nerve dysfunction, specifically CN IX (glossopharyngeal) and CN X (vagus). These nerves control the muscles of the soft palate, pharynx, and larynx. The key clinical findings here point to unilateral vagus nerve dysfunction. The uvula deviates to the left because the right side of the soft palate is weak - when healthy muscles on the left contract normally while the right side fails to contract, the uvula gets pulled toward the functioning (left) side. The diminished gag reflex on the right confirms right-sided CN IX/X dysfunction, while the nasal voice and choking indicate weakness of pharyngeal and soft palate muscles. The nucleus ambiguus in the medulla contains the motor neurons for CN IX and CN X that innervate these muscles. A lesion in the right nucleus ambiguus (A) would cause right-sided weakness, explaining all the findings. Choice B (left corticobulbar fibers) is incorrect because these upper motor neurons have bilateral innervation to the nucleus ambiguus, so a unilateral lesion wouldn't cause the clear asymmetry seen here. Choice C (right hypoglossal nucleus) controls tongue movement, not soft palate function - you'd see tongue deviation instead. Choice D (left solitary nucleus) processes sensory information and wouldn't cause the motor weakness described. Remember: uvula deviation follows the "rule of pointing away from the lesion" - it points toward the normal, functioning side. This helps localize brainstem lesions affecting cranial nerves IX and X.

Question 12

1. Left posterolateral putamen receiving corticostriatal input
2. Right subthalamic nucleus of the basal ganglia (correct answer)
3. Left ventrolateral nucleus of the thalamus
4. Right dentate nucleus of the cerebellum

Explanation: When you encounter sudden-onset involuntary movements after a stroke, you're dealing with damage to the basal ganglia circuitry that normally controls and modulates movement. The key here is recognizing the specific movement pattern and understanding the cross-over anatomy. Hemiballismus—the wild, flinging movements described—is a classic sign of subthalamic nucleus damage. The subthalamic nucleus normally provides excitatory input to the globus pallidus, which helps suppress unwanted movements. When the subthalamic is damaged, this brake on movement is released, causing the characteristic ballistic movements. Since the patient has left-sided movements, the damage must be in the right subthalamic nucleus due to the crossing of motor pathways. Answer choice A (left posterolateral putamen) is incorrect because putamen damage typically causes different movement disorders like chorea or dystonia, not hemiballismus. The laterality is also wrong. Answer choice C (left ventrolateral thalamus) is anatomically on the wrong side and functionally incorrect—thalamic damage here would more likely cause sensory deficits or different motor problems. Answer choice D (right dentate nucleus) involves cerebellar circuitry, which would cause ataxia, intention tremor, or coordination problems, not the explosive ballistic movements seen here. Remember this pattern: sudden-onset hemiballismus after stroke = contralateral subthalamic nucleus damage. The subthalamic nucleus is particularly vulnerable to small vessel strokes due to its blood supply, making this a high-yield association for Step 1.

Question 13

1. Left medial medulla containing the pyramids and medial lemniscus (correct answer)
2. Right medial medulla containing the pyramid and medial lemniscus
3. Left lateral medulla containing the inferior cerebellar peduncle
4. Right paramedian pons containing corticospinal fibers

Explanation: When you encounter a patient with crossed neurological signs (symptoms on opposite sides of the body), think about brainstem lesions where cranial nerve nuclei and ascending/descending tracts are in close proximity before they cross. This patient has right-sided motor weakness and sensory loss with left tongue deviation - a classic medial medullary syndrome pattern. The hypoglossal nerve (CN XII) controls tongue protrusion, and damage to the left hypoglossal nucleus or fibers causes the tongue to deviate toward the affected (left) side due to unopposed action of the right genioglossus muscle. The right-sided motor and sensory deficits indicate damage to descending motor fibers and ascending sensory pathways before they cross. Answer A correctly identifies the left medial medulla, where the pyramidal tract (motor fibers that will cross at the pyramidal decussation) and medial lemniscus (discriminative touch and vibration fibers that already crossed at the medulla) travel alongside the hypoglossal nucleus and fibers. Answer B places the lesion on the right side, but this would cause right tongue deviation and left-sided body symptoms. Answer C describes lateral medullary syndrome (Wallenberg syndrome), which presents with different symptoms including dysphagia, vertigo, and ipsilateral facial sensory loss. Answer D involves the pons, which wouldn't affect the hypoglossal nerve that originates in the medulla. Remember: In medial medullary syndrome, the "rule of 4s" helps - think hypoglossal nerve plus motor/sensory tracts, with the tongue pointing toward the lesion side while body symptoms appear contralaterally.

Question 14

1. Central chemoreceptors on the ventrolateral surface of the medulla (correct answer)
2. Peripheral chemoreceptors in the carotid body projecting via glossopharyngeal nerve
3. Pontine pneumotaxic center modulating inspiratory cutoff
4. Dorsal respiratory group in the nucleus tractus solitarius

Explanation: When you encounter questions about respiratory control at high altitude, focus on distinguishing between acute versus chronic adaptations and which receptors drive each response. At high altitude, the immediate hyperventilation response occurs because peripheral chemoreceptors (carotid bodies) detect decreased oxygen partial pressure and stimulate breathing via the glossopharyngeal nerve. However, this hyperventilation causes respiratory alkalosis, which would normally inhibit further breathing. The key adaptation over "ensuing days" involves central chemoreceptors. Central chemoreceptors on the ventrolateral surface of the medulla (A) are the correct answer because they directly sense cerebrospinal fluid pH changes and drive the chronic ventilatory adaptation. Initially, the alkalotic CSF pH from hyperventilation would suppress breathing, but over days, the blood-brain barrier actively transports bicarbonate out of the CSF, restoring normal CSF pH despite continued hyperventilation. This allows sustained increased ventilation at altitude. Option B describes peripheral chemoreceptors, which initiate the acute response to hypoxia but don't sense CSF pH changes. Option C, the pneumotaxic center, modulates breathing patterns but doesn't directly sense pH - it's involved in fine-tuning inspiratory duration. Option D, the dorsal respiratory group, integrates sensory input but the actual pH sensing occurs at the ventrolateral medullary chemoreceptors, not in the nucleus tractus solitarius. Remember: Central chemoreceptors = CSF pH sensing = chronic adaptation. Peripheral chemoreceptors = blood oxygen sensing = acute response. The "ensuing days" timeframe is your clue that chronic central adaptation is being tested.

Question 15

1. Right cavernous sinus affecting cranial nerves III, IV, V1, and VI (correct answer)
2. Right orbital apex compressing cranial nerves II, III, and VI
3. Right superior orbital fissure selectively injuring cranial nerve VI
4. Right Edinger–Westphal nucleus in the midbrain tegmentum

Explanation: When you encounter a diabetic patient with cranial nerve deficits, think systematically about anatomical locations where multiple cranial nerves travel together. The combination of findings here—ptosis, mydriasis, ophthalmoplegia, and decreased corneal reflex—suggests involvement of cranial nerves III, IV, V1, and VI. The cavernous sinus is the key location where these nerves converge. CN III (oculomotor) causes ptosis and pupillary constriction when intact, so its dysfunction produces ptosis and mydriasis. CN IV (trochlear) and CN VI (abducens) control eye movement, explaining the ophthalmoplegia and diplopia. The ophthalmic division of CN V (V1) carries corneal sensation, accounting for the decreased corneal reflex while facial sensation remains intact (since V2 and V3 don't traverse the cavernous sinus). Option A correctly identifies this cavernous sinus syndrome affecting all relevant cranial nerves. Option B (orbital apex) is wrong because CN IV doesn't pass through the orbital apex, and this location wouldn't explain the corneal reflex loss. Option C (superior orbital fissure with selective CN VI injury) is incorrect because isolated CN VI lesions wouldn't cause ptosis, mydriasis, or corneal reflex changes. Option D (Edinger-Westphal nucleus) is wrong because a midbrain lesion wouldn't affect CN IV, V1, or VI, which have different anatomical courses. Remember that diabetic patients are prone to cavernous sinus thrombosis and other vascular complications. When you see multiple cranial nerve palsies in a diabetic, always consider the cavernous sinus as the most likely anatomical correlate.

Question 16

1. Conus medullaris at the L1 vertebral level containing sacral spinal segments
2. Cauda equina nerve roots within the lumbar cistern below L2 (correct answer)
3. Anterior horn cells in the lumbar enlargement segments L2–S2
4. Posterior columns of the thoracic spinal cord carrying vibration sensation

Explanation: When you encounter a patient with spinal cord compression, the key is matching the clinical presentation to the specific anatomical location affected. This patient's combination of lower limb weakness, saddle anesthesia (numbness in the perineal area), urinary retention, and absent reflexes points to a specific lesion pattern. The constellation of findings here - bilateral lower extremity weakness with saddle anesthesia and bladder dysfunction - is classic for cauda equina syndrome. The cauda equina consists of nerve roots from L2 through S5 that travel within the lumbar cistern below the termination of the spinal cord at L1-L2. These nerve roots control lower extremity motor function, bowel/bladder function, and sensation in the saddle distribution. Cancer patients, particularly those with small-cell lung carcinoma, are at high risk for vertebral metastases that can compress these nerve roots. Choice A is incorrect because conus medullaris syndrome typically presents with mixed upper and lower motor neuron signs, not the pure lower motor neuron pattern seen here. Choice C is wrong because anterior horn cell involvement would cause motor symptoms without the sensory loss and bladder dysfunction. Choice D is incorrect because posterior column involvement would primarily affect proprioception and vibration sense, not motor function or bladder control. Remember that cauda equina syndrome is a neurological emergency requiring immediate decompression. On the USMLE, saddle anesthesia combined with bladder dysfunction should immediately make you think of cauda equina compression, especially in cancer patients where vertebral metastases are common.

Question 17

1. Dorsal midbrain at the level of the superior colliculus (Parinaud region) (correct answer)
2. Paramedian pontine reticular formation controlling horizontal gaze
3. Medial longitudinal fasciculus in the mid-pons
4. Pretectal nuclei of the anterior diencephalon mediating pupillary light reflex

Explanation: When you encounter a question combining eye movement disorders with pupillary abnormalities, think about anatomical localization in the brainstem, particularly the midbrain where these functions converge. This patient presents with classic Parinaud syndrome (dorsal midbrain syndrome). The key features are impaired downward gaze and light-near dissociation (pupils don't react to light but still accommodate). The vertical diplopia worsening with downward activities like reading or descending stairs specifically points to supranuclear control problems for vertical eye movements. The dorsal midbrain at the superior colliculus level (answer A) houses the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF), which controls vertical gaze, and the posterior commissure, crucial for downward gaze. Lesions here also affect the pretectal area, disrupting the pupillary light reflex while sparing accommodation pathways that travel more ventrally. Answer B is incorrect because the paramedian pontine reticular formation controls horizontal, not vertical, gaze movements. Answer C, the medial longitudinal fasciculus in mid-pons, would cause internuclear ophthalmoplegia with horizontal diplopia and convergence problems, not vertical gaze palsy. Answer D, while the pretectal nuclei do mediate pupillary light reflex, this location alone wouldn't explain the vertical gaze palsy - you need the more comprehensive dorsal midbrain lesion. Remember: Light-near dissociation plus vertical gaze problems equals dorsal midbrain pathology. This pattern appears frequently on USMLE Step 1, so associate Parinaud syndrome with pineal region tumors, hydrocephalus, or other dorsal midbrain lesions.

Question 18

1. Left posterior superior temporal gyrus (Wernicke area)
2. Left inferior frontal gyrus (Broca area)
3. Left arcuate fasciculus connecting temporal and frontal language areas (correct answer)
4. Right inferior parietal lobule homologous to Wernicke area

Explanation: When you encounter a patient with fluent speech, paraphasic errors, intact comprehension, but inability to repeat, you're dealing with a classic pattern called conduction aphasia. This specific combination of symptoms points to a disconnection between language areas rather than damage to the primary language centers themselves. The key insight is that repetition requires information to flow from Wernicke's area (where sounds are processed and understood) to Broca's area (where speech is produced). The arcuate fasciculus is the white matter tract that connects these regions. When it's damaged, patients can understand speech (Wernicke's area intact) and speak fluently (Broca's area intact), but cannot repeat because the connection between understanding and production is severed. Choice A (Wernicke's area damage) would cause fluent aphasia but with poor comprehension - the opposite of what's described. Choice B (Broca's area damage) would result in non-fluent, effortful speech with good comprehension and repetition difficulties, but the speech pattern here is fluent. Choice D (right hemisphere damage) wouldn't typically cause the classic language deficits described, as language is predominantly left-lateralized in right-handed individuals. Remember this pattern: fluent speech + good comprehension + poor repetition = conduction aphasia = arcuate fasciculus damage. The inability to repeat despite intact input (comprehension) and output (fluent speech) is the hallmark of a disconnection syndrome affecting the pathway between language areas.

Question 19

1. Right cerebellar hemisphere involving the lateral cortex (correct answer)
2. Midline vermis of the cerebellum
3. Left red nucleus of the midbrain
4. Right inferior olivary nucleus of the medulla

Explanation: When you encounter cerebellar symptoms, think about anatomical localization based on the specific deficits present. The cerebellum has distinct functional regions: the lateral hemispheres control limb coordination, while the midline vermis controls truncal stability and gait. This patient shows classic signs of lateral cerebellar hemisphere dysfunction. The inability to rapidly alternate pronation and supination demonstrates dysdiadochokinesia, and the intention tremor during finger-to-nose testing indicates dysmetria - both hallmarks of ipsilateral cerebellar hemisphere lesions. Crucially, his speech is normal and there's no truncal ataxia, which rules out midline involvement. Choice A is correct because right-sided limb symptoms with preserved truncal function precisely localizes to the right cerebellar hemisphere's lateral cortex, which controls ipsilateral limb coordination. Choice B is wrong because midline vermis lesions cause truncal ataxia, gait instability, and dysarthria - none of which this patient exhibits. The vermis doesn't control limb diadochokinesis. Choice C is incorrect because red nucleus lesions typically cause contralateral tremor at rest (rubral tremor), not the ipsilateral intention tremor and dysdiadochokinesia seen here. The red nucleus is part of the extrapyramidal system, not the cerebellar circuit. Choice D is wrong because inferior olivary nucleus lesions cause palatal myoclonus and hypertrophic olivary degeneration, not the appendicular cerebellar signs described. Remember: cerebellar hemisphere lesions cause ipsilateral limb coordination problems, while vermis lesions cause midline/truncal problems. The cerebellum's anatomy directly correlates with its clinical presentations.

Question 20

1. Right lateral corticospinal tract at T10 (correct answer)
2. Left posterior column–medial lemniscus pathway at T10
3. Left lateral spinothalamic tract at T10
4. Right anterior white commissure at T8

Explanation: When you encounter spinal cord injury questions, focus on the anatomical organization of ascending and descending tracts and understand that damage typically affects structures on the same side (ipsilateral) as the injury. This patient presents with Brown-Séquard syndrome (hemisection of the spinal cord). The key finding is ipsilateral motor weakness (right side) with contralateral pain/temperature loss (left side) and ipsilateral vibration/proprioception loss (right side). The motor weakness tells you which side is damaged - since the corticospinal tract doesn't cross until the medulla, damage above that crossing point affects the same side as the lesion. The right lateral corticospinal tract at T10 (Answer A) explains the complete motor loss in the right lower limb. This tract carries motor fibers that have already crossed in the medulla, so a right-sided lesion causes right-sided weakness. The hemisection also damages the right posterior columns (causing right-sided vibration/proprioception loss) and left spinothalamic fibers that have crossed from the left side (causing left-sided pain/temperature loss). Answer B is wrong because left posterior column damage would cause left-sided, not right-sided, vibration/proprioception loss. Answer C is incorrect because left spinothalamic tract damage would cause right-sided, not left-sided, pain/temperature loss. Answer D doesn't explain the motor findings and would affect bilateral pain/temperature sensation. Remember: In Brown-Séquard syndrome, motor weakness and vibration/proprioception loss occur ipsilateral to the lesion, while pain/temperature loss occurs contralateral. The side with motor weakness tells you which side is damaged.