A 45-year-old man presents with contralateral hemiplegia and ipsilateral CN III palsy following a midbrain infarct. Which tract is most likely interrupted to cause the contralateral motor deficit?
- A Rubrospinal tract in the tegmentum
- B Tectospinal tract in the tectum
- C Corticospinal tract in the crus cerebri ✓
- D Reticulospinal tract in the ventral midbrain
Correct answer: C. Corticospinal tract in the crus cerebri
Explanation
The combination of ipsilateral CN III palsy and contralateral hemiplegia is termed Weber syndrome, caused by a lesion in the midbrain basis pedunculi. The corticospinal and corticobulbar fibers occupy the middle three-fifths of the crus cerebri; interruption here produces contralateral upper motor neuron weakness. The oculomotor nerve fascicles pass through the adjacent tegmentum, explaining the ipsilateral CN III deficit.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.