A patient presents with contralateral hemiplegia, ipsilateral oculomotor palsy (CN III palsy) with a dilated pupil, and ptosis. The lesion is localized to:

• A Basis pontis (Millard-Gubler syndrome)
• B Lateral medulla (Wallenberg syndrome)
• C Posterior limb of the internal capsule
• D Midbrain crus cerebri (Weber's syndrome) ✓

Explanation

Weber's syndrome results from an infarct in the base of the midbrain (crus cerebri / basis pedunculi), involving the corticospinal and corticobulbar tracts along with the exiting fascicles of the oculomotor nerve (CN III). This produces ipsilateral CN III palsy (ptosis, dilated pupil, 'down-and-out' eye) combined with contralateral hemiplegia — a classic crossed brainstem syndrome. Millard-Gubler syndrome involves the basis pontis with CN VI and VII ipsilaterally plus contralateral hemiplegia.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.