USMLE Step 1 Quiz: Neuroanatomy And Localization
20 questions · exam conditions
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Neuroanatomy And LocalizationQuestion 1 of 20

Loss of pain/temperature on the right face and left limbs, with right miosis and hoarse voice. Where is the lesion?

Right lateral medulla
Left lateral medulla
Right lateral pons
Left medial medulla
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USMLE Step 1 Quiz

USMLE Step 1 Quiz: Neuroanatomy And Localization

Practice Neuroanatomy And Localization in USMLE Step 1 with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

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.

All questions

Question 1

Loss of pain/temperature on the right face and left limbs, with right miosis and hoarse voice. Where is the lesion?

  1. Right lateral medulla (correct answer)
  2. Left lateral medulla
  3. Right lateral pons
  4. Left medial medulla
Explanation: Pain/temperature loss on the right face is ipsilateral because the spinal trigeminal tract hasn't crossed, while left limb loss is contralateral from spinothalamic tract damage. Right miosis indicates ipsilateral Horner, and hoarseness points to nucleus ambiguus. This combination is right lateral medullary (Wallenberg) syndrome. The tempting wrong answer, left lateral medulla, would cause left face with right limb loss, the opposite pattern.

Question 2

Right gaze: left eye fails to adduct, right eye nystagmus; convergence intact. Lesion site?

  1. Left MLF lesion (correct answer)
  2. Right MLF lesion
  3. Left CN VI lesion
  4. Right CN III lesion
Explanation: For right gaze, the right sixth nerve abducts the right eye, and internuclear fibers cross through the left MLF to activate the left medial rectus. A left MLF lesion blocks that signal, so the left eye fails to adduct while the right eye abducts with nystagmus; convergence stays intact because the third nerve nucleus is not damaged. The tempting wrong answer is a right MLF lesion, but that would instead impair right eye adduction on left gaze.

Question 3

Upgaze palsy, pupils constrict with accommodation but not to light, lid retraction. Lesion site?

  1. Ventral midbrain
  2. Dorsal midbrain (correct answer)
  3. Lateral pons
  4. Medial medulla
Explanation: Upgaze palsy, light-near dissociation, and lid retraction (Collier sign) point to a dorsal midbrain (tectal) lesion at the superior colliculus/posterior commissure, called Parinaud syndrome. The most tempting wrong answer is ventral midbrain, but that causes CN III palsy with a dilated pupil and contralateral hemiparesis, not preserved accommodation with lid retraction.

Question 4

Right eye is down and out with a fixed dilated pupil; left-limb tremor. Lesion site?

  1. Left cerebral peduncle
  2. Left midbrain tegmentum
  3. Right cerebral peduncle
  4. Right midbrain tegmentum (correct answer)
Explanation: Right eye down and out with a fixed dilated pupil is a complete CN III palsy, localizing to the right midbrain. The left-limb tremor points to the adjacent right red nucleus in the tegmentum. The tempting cerebral peduncle lesion would cause contralateral hemiparesis, not tremor, and would not explain the red nucleus sign.

Question 5

Symmetric saddle anesthesia, early urinary retention, normal leg strength. Lesion site?

  1. Thoracic cord lesion
  2. Cauda equina lesion
  3. Conus medullaris lesion (correct answer)
  4. Sacral plexus lesion
Explanation: Saddle anesthesia with early urinary retention but preserved leg strength points to conus medullaris involvement. The conus holds sacral segments controlling perineal sensation and bladder, while leg fibers are higher and remain intact. The tempting wrong answer is cauda equina lesion, but that produces asymmetric radicular pain, leg weakness, and later sphincter signs.

Question 6

A 63F with acute confusion and fluent aphasia; MRI shows left posterior temporal infarct (inferior division MCA). Which structure is affected?

  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 7

A 33M with right-sided weakness and loss of vibration below C5 after hemicord injury; MRI shows right C5 lesion. What diagnosis fits?

  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 8

A 24M after humeral shaft fracture has wrist drop and dorsal hand numbness; X-ray shows midshaft fracture. Which nerve is injured?

  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 9

A 52F with ptosis, mydriasis, and down-and-out right eye after posterior communicating aneurysm. Which cranial nerve is involved?

  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 10

A 57F with sudden right facial paralysis including forehead and loss of taste anterior tongue; MRI shows facial canal enhancement. Which nerve is involved?

  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 11

A 45M with sudden binocular diplopia and inability to abduct right eye; MRI shows pontine lesion. Which cranial nerve is involved?

  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 12

Cannot look left; eyes rest deviated right. Cold water in left ear drives eyes left. Lesion?

  1. Right frontal eye field (correct answer)
  2. Left frontal eye field
  3. Left pontine gaze center
  4. Right MLF infarction
Explanation: Cold water in the left ear drives the eyes left, so the brainstem left-gaze pathway is intact and the lesion is supranuclear. Loss of leftward gaze with the eyes resting deviated right localizes to the right frontal eye field. The tempting trap is the left pontine gaze center, which also prevents left gaze and deviates the eyes right, but it would block caloric-induced leftward eye movement.

Question 13

Right ptosis, dilated pupil, left intention tremor, no weakness. Lesion?

  1. Right red nucleus (correct answer)
  2. Left red nucleus
  3. Right crus cerebri
  4. Left crus cerebri
Explanation: Right ptosis with dilated pupil indicates right CN III palsy. Left intention tremor without weakness localizes to the right red nucleus, which affects crossing cerebellar fibers for contralateral coordination and the nearby right CN III fascicles. Right crus cerebri would cause left hemiparesis; left red nucleus would cause left CN III palsy and right-sided tremor.

Question 14

Right painful ophthalmoplegia, proptosis, V2 numbness, normal vision. Lesion?

  1. Left cavernous sinus lesion
  2. Right cavernous sinus lesion (correct answer)
  3. Right orbital apex lesion
  4. Right superior orbital fissure
Explanation: Ophthalmoplegia plus proptosis plus V2 numbness with normal vision localizes to the right cavernous sinus, where CN III, IV, V1, V2, and VI course; proptosis results from venous congestion. An orbital apex lesion is the main trap because it can also cause painful ophthalmoplegia and proptosis, but it would compress the optic nerve and reduce vision, which is normal here.

Question 15

Sudden flinging of right arm and leg; MRI shows infarct in which structure?

  1. Right globus pallidus interna
  2. Right subthalamic nucleus
  3. Left globus pallidus interna
  4. Left subthalamic nucleus (correct answer)
Explanation: Sudden flinging of one side is hemiballismus, caused by damage to the contralateral subthalamic nucleus. Because the right arm and leg are moving, the lesion is in the left hemisphere, so the left subthalamic nucleus. The globus pallidus interna is tempting because it is in the basal ganglia, but it is not the structure whose damage causes hemiballismus.

Question 16

Right facial pain and left body pain/temperature loss, right Horner, dysphagia. Lesion?

  1. Right medial medulla
  2. Left lateral medulla
  3. Right lateral medulla (correct answer)
  4. Right pontine tegmentum
Explanation: Facial pain and Horner are ipsilateral to the lateral medulla, and dysphagia localizes to the ipsilateral nucleus ambiguus; spinothalamic pain/temperature loss is contralateral. Right face/Horner/dysphagia with left body pain/temperature loss places the lesion in the right lateral medulla. The tempting wrong choice is left lateral medulla, but that would produce left facial pain and right Horner.

Question 17

A 62M with sudden vertigo, dysphagia, hoarseness, and right facial pain with left body pain loss; MRI shows PICA infarct. Which structure is affected?

  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 18

A 56-year-old woman suddenly develops dizziness, hoarseness, difficulty swallowing, and loss of pain and temperature sensation on the right side of her face and the left side of her body. Examination shows nystagmus and ataxia of the right arm. Which of the following arterial territories is most likely infarcted?

  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 19

A 67-year-old man with hypertension is found to have conjugate deviation of the eyes toward the right side after a lacunar infarct. Pupils are equal and reactive; vestibulo-ocular reflex is preserved. Motor and sensory examinations are otherwise normal. The lesion most likely involves which of the following structures?

  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 20

A 29-year-old woman is evaluated for progressive visual difficulty. Confrontation testing shows loss of peripheral vision in both eyes, sparing central fields. Pupils react normally, and funduscopic examination is unremarkable. Where is the lesion most likely located?

  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.