NBME Neuroscience 3 — Questions and Answers
Question 1: A patient loses fine touch, vibration, and proprioception below the level of a thoracic spinal cord lesion, bilaterally. Which tract is most likely damaged?
- Lateral spinothalamic tract
- Dorsal columns (posterior funiculus) (Correct answer)
- Anterior spinothalamic tract
- Spinocerebellar tract
Correct answer: Dorsal columns (posterior funiculus)
The dorsal columns (fasciculus gracilis and cuneatus) carry ipsilateral fine touch, vibration, and proprioception; bilateral loss indicates bilateral dorsal column damage.
Question 2: A patient sustains a left hemisection of the thoracic spinal cord (Brown-Séquard). Which finding is expected below the lesion?
- Left-sided loss of pain/temperature; right-sided loss of proprioception
- Left-sided loss of proprioception; right-sided loss of pain/temperature (Correct answer)
- Bilateral loss of all sensory modalities
- Right-sided spastic paralysis with left-sided proprioceptive loss
Correct answer: Left-sided loss of proprioception; right-sided loss of pain/temperature
Brown-Séquard: ipsilateral (left) UMN weakness and dorsal column loss because those tracts are uncrossed at that level; contralateral (right) spinothalamic (pain/temp) loss because the STT crosses within 1-2 spinal levels.
Question 3: Which dopaminergic pathway is most associated with the positive symptoms of schizophrenia?
- Nigrostriatal pathway
- Mesolimbic pathway (Correct answer)
- Mesocortical pathway
- Tuberoinfundibular pathway
Correct answer: Mesolimbic pathway
Hyperactivity of the mesolimbic dopaminergic pathway (VTA to nucleus accumbens/limbic areas) underlies the positive symptoms (hallucinations, delusions) of schizophrenia.
Question 4: An ascending demyelinating polyneuropathy with albuminocytologic dissociation on CSF analysis and autonomic instability is most consistent with which diagnosis?
- Multiple sclerosis
- Guillain-Barré syndrome (Correct answer)
- Amyotrophic lateral sclerosis
- Charcot-Marie-Tooth disease
Correct answer: Guillain-Barré syndrome
Guillain-Barré syndrome is an autoimmune demyelinating polyradiculoneuropathy with ascending weakness, areflexia, albuminocytologic dissociation (high protein, normal cells), and potential autonomic dysfunction.
Question 5: A patient presents with fasciculations, muscle atrophy, hyperreflexia, and a Babinski sign in the same extremities. Which diagnosis best explains both upper and lower motor neuron signs?
- Syringomyelia
- Amyotrophic lateral sclerosis (ALS) (Correct answer)
- Multiple sclerosis
- Subacute combined degeneration
Correct answer: Amyotrophic lateral sclerosis (ALS)
ALS selectively destroys both UMNs (cortex, corticospinal tract) and LMNs (anterior horn cells, cranial motor nuclei), producing simultaneous hyperreflexia/spasticity and fasciculations/atrophy.
Question 6: During synaptic transmission at the neuromuscular junction, which event directly triggers vesicle fusion and acetylcholine release?
- Sodium influx through voltage-gated channels
- Calcium influx through voltage-gated P/Q-type channels (Correct answer)
- Potassium efflux causing membrane repolarization
- cAMP activation of protein kinase A
Correct answer: Calcium influx through voltage-gated P/Q-type channels
Action potential depolarization opens presynaptic voltage-gated Ca²⁺ (P/Q-type) channels; the resulting Ca²⁺ influx triggers SNARE-mediated vesicle fusion and ACh exocytosis.
Question 7: Which neurotransmitter is the primary inhibitory neurotransmitter in the brain, and benzodiazepines enhance its effect at which receptor type?
- Glycine; GABA-B receptor
- GABA; GABA-A receptor (Cl⁻ channel) (Correct answer)
- Glutamate; NMDA receptor
- Dopamine; D2 receptor
Correct answer: GABA; GABA-A receptor (Cl⁻ channel)
GABA is the principal inhibitory CNS neurotransmitter; benzodiazepines bind the GABA-A receptor (a ligand-gated Cl⁻ channel) and increase frequency of Cl⁻ channel opening.
NBME Neuroscience 3
A patient loses fine touch, vibration, and proprioception below the level of a thoracic spinal cord lesion, bilaterally.
Which tract is most likely damaged?