Localization And Neurologic Examination - USMLE Step 2

1. This specific visual field defect is most likely caused by an occlusion of which artery?
   Answer: Left posterior cerebral artery A right homonymous hemianopia localizes the lesion to the left optic tract, optic radiation, or primary visual cortex. The presence of macular sparing is a key localizing sign, suggesting an infarct in the primary visual cortex located in the occipital lobe. The occipital lobe is primarily supplied by the posterior cerebral artery (PCA). Macular sparing occurs because the macular area of the visual cortex often has a dual blood supply from both the PCA and the middle cerebral artery (MCA). Therefore, a left PCA occlusion is the most likely cause.
2. Which of the following is the most likely site of nerve compression?
   Answer: Carpal tunnel at the wrist This patient's symptoms and signs are classic for carpal tunnel syndrome, which is caused by compression of the median nerve as it passes through the carpal tunnel at the wrist. The sensory distribution (first 3.5 digits) and motor weakness (thenar muscles) are specific to the median nerve. The positive Tinel sign (tapping on the nerve) further supports this diagnosis.
3. The lesion is most likely located at which of the following sites?
   Answer: Ulnar nerve at the elbow This patient has a classic ulnar neuropathy, with weakness of intrinsic hand muscles (interossei, hypothenar) leading to a 'claw hand' deformity, and sensory loss in the ulnar distribution. The most common site of ulnar nerve compression is the cubital tunnel at the elbow. Sparing of sensation over the medial forearm (innervated by the medial antebrachial cutaneous nerve, which branches off the medial cord) helps to distinguish this from a more proximal brachial plexus lesion. A lesion at Guyon's canal would typically spare the ulnar-innervated muscles in the forearm.
4. What is the most likely site of the lesion?
   Answer: Anterior spinal cord This clinical picture is characteristic of anterior cord syndrome. The lesion affects the anterior two-thirds of the spinal cord, damaging the corticospinal tracts (causing bilateral spastic weakness below the lesion) and the spinothalamic tracts (causing bilateral loss of pain and temperature). The dorsal columns, which transmit vibration and proprioception, are spared because they are located in the posterior cord and have a separate blood supply (posterior spinal arteries).
5. Which of the following locations best explains this patient's presentation?
   Answer: Right lateral medulla This is a classic presentation of Wallenberg syndrome, or lateral medullary syndrome, typically caused by an occlusion of the posterior inferior cerebellar artery (PICA). The lesion in the right lateral medulla affects the spinal trigeminal nucleus (ipsilateral face sensation), spinothalamic tract (contralateral body sensation), nucleus ambiguus (dysphagia, hoarseness), inferior cerebellar peduncle (ipsilateral ataxia), and descending sympathetic fibers (ipsilateral Horner syndrome).
6. Where is the infarct most likely located?
   Answer: Left inferior frontal gyrus The patient's clinical presentation is consistent with Broca's aphasia, an expressive aphasia characterized by non-fluent, agrammatical speech with intact comprehension. This localizes the lesion to Broca's area, which is located in the inferior frontal gyrus of the dominant (usually left) hemisphere. The proximity of this area to the primary motor cortex explains the associated contralateral face and arm weakness.
7. Where is the lesion most likely located?
   Answer: Left temporal lobe This patient has fluent (Wernicke's) aphasia, characterized by impaired comprehension and fluent but meaningless speech. This localizes to Wernicke's area in the dominant (usually left) superior temporal gyrus. The associated right superior quadrantanopia ('pie in the sky') is caused by involvement of the lower optic radiation (Meyer's loop), which also passes through the temporal lobe.
8. Which of the following locations best explains this constellation of findings?
   Answer: Left cavernous sinus This patient has multiple cranial neuropathies affecting the left eye. The ptosis and 'down and out' gaze indicate a CN III palsy. The numbness over the forehead and cheek corresponds to the V1 and V2 divisions of the trigeminal nerve. The cavernous sinus is a unique anatomical space through which CN III, IV, VI, V1, and V2 travel. A single lesion here, such as from pituitary apoplexy or thrombosis, can affect all these nerves simultaneously.
9. A lesion in which of the following locations would best explain her symptoms?
   Answer: Left facial nerve nucleus or nerve This patient has a left-sided lower motor neuron (LMN) facial palsy, involving both the upper (forehead) and lower face. This localizes the lesion to the facial nerve nucleus in the pons or the peripheral facial nerve (CN VII) itself after it exits the brainstem. In contrast, an upper motor neuron (UMN) lesion, such as in the motor cortex or internal capsule, would spare the forehead due to bilateral cortical innervation of the upper face.
10. Which of the following is the most likely location of the lesion responsible for this patient's symptoms?
    Answer: Left cerebral peduncle of the midbrain This patient presents with a left oculomotor nerve (CN III) palsy (ptosis, 'down and out' gaze, mydriasis) and contralateral (right-sided) hemiparesis. This constellation of findings is characteristic of Weber syndrome, which is caused by a lesion in the left cerebral peduncle of the midbrain, affecting the exiting CN III fascicles and the adjacent corticospinal tract.
11. A lesion affecting which of the following nerve roots is the most likely cause?
    Answer: L5 The patient's symptoms localize to the L5 nerve root. Weakness of foot dorsiflexion and great toe extension is primarily an L5 myotome function. The sensory loss on the dorsum of the foot is consistent with the L5 dermatome. The ankle reflex (S1) is preserved, and weakness of knee extension (L4) is absent, making L5 the most likely affected root.
12. What is the most likely location of this patient's lesion?
    Answer: Right parietal lobe This patient is demonstrating contralateral hemispatial neglect, a condition where she is unaware of or fails to attend to stimuli on the left side of her body and space. This is a classic sign of a lesion in the non-dominant (usually right) parietal lobe, which is responsible for spatial awareness and attention.
13. Which of the following is the most likely site of injury?
    Answer: Central cervical spinal cord This patient's presentation is classic for central cord syndrome. This syndrome is often caused by a hyperextension injury, leading to damage to the central part of the cervical spinal cord. This affects the spinothalamic fibers crossing in the anterior commissure (causing the 'cape-like' sensory loss) and the medial aspects of the corticospinal tracts, which control the arms, leading to upper extremity weakness that is greater than lower extremity weakness.
14. A lesion in which of the following structures is most likely responsible for his symptoms?
    Answer: Right subthalamic nucleus The patient is exhibiting hemiballismus, a hyperkinetic movement disorder characterized by violent, flinging movements of the limbs on one side of the body. This condition is most commonly caused by a contralateral lesion in the subthalamic nucleus, which is part of the basal ganglia motor circuit. A small lacunar stroke is a common etiology.
15. Which nerve is most likely injured?
    Answer: Radial nerve The clinical presentation of 'wrist drop' (inability to extend the wrist and fingers) combined with sensory loss on the posterior forearm and dorsolateral hand is pathognomonic for a radial nerve injury. The radial nerve runs in the spiral groove of the humerus, making it particularly vulnerable to injury with midshaft humeral fractures.
16. Where is the lesion most likely located?
    Answer: Right cerebellar hemisphere The patient's signs of ipsilateral limb ataxia (dysmetria, intention tremor) and truncal ataxia (falling to the right) are classic for a lesion in the right cerebellar hemisphere. The cerebellum is responsible for coordinating voluntary movements, posture, balance, and motor learning. Cerebellar lesions cause deficits on the same side of the body.
17. Which of the following is the most likely location of the lesion?
    Answer: Right spinal cord at C5 This patient's findings are classic for Brown-Séquard syndrome (spinal cord hemisection). The lesion is on the right side at the C5 level, causing ipsilateral (right-sided) motor weakness (corticospinal tract) and loss of proprioception/vibration (dorsal columns), and contralateral (left-sided) loss of pain and temperature (spinothalamic tract, which decussates at the level of the cord).
18. This presentation is most characteristic of a lacunar infarct in which of the following locations?
    Answer: Right posterior limb of the internal capsule This is a classic 'pure motor stroke,' characterized by contralateral hemiparesis affecting the face, arm, and leg equally, without any sensory, visual, or cognitive deficits. This syndrome is caused by a small, deep lacunar infarct in the posterior limb of the internal capsule, where the corticospinal and corticobulbar motor fibers are densely packed. A cortical stroke would typically show disproportionate weakness (e.g., face/arm more than leg) and would likely have associated cortical signs (aphasia, neglect).
19. This patient's signs are best explained by a lesion in which of the following structures?
    Answer: Right medial longitudinal fasciculus (MLF) This patient has internuclear ophthalmoplegia (INO) affecting the right eye's adduction. INO is caused by a lesion in the medial longitudinal fasciculus (MLF), a white matter tract that coordinates horizontal eye movements by connecting the contralateral abducens nucleus (CN VI) with the ipsilateral oculomotor nucleus (CN III). A lesion in the right MLF prevents the right medial rectus from firing when the left eye abducts, causing impaired adduction of the right eye on leftward gaze. The nystagmus in the abducting eye is also characteristic. Convergence is spared as it does not rely on the MLF.
20. An MRI of the brain would most likely show an infarct in which location?
    Answer: Left posterolateral thalamus This patient presents with a combined sensorimotor stroke affecting the left posterolateral thalamus. The thalamus contains both the ventral posterior lateral (VPL) and ventral posterior medial (VPM) nuclei, which relay sensory information, as well as motor relay nuclei. A strategic infarct in this region can cause both contralateral hemiparesis and complete sensory loss affecting all modalities on the face, arm, and leg. The absence of cortical signs (aphasia, neglect) helps localize this to a subcortical structure.

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