A 58-year-old hypertensive man develops horizontal diplopia. Examination shows complete failure of conjugate gaze towards the left, failure of adduction of the left eye on right gaze, and abduction nystagmus of the right eye on right gaze. The only preserved horizontal movement is abduction of the right eye. The lesion is located in the:

• A Left pontine tegmentum involving the left PPRF and left MLF ✓
• B Right pontine tegmentum involving the right PPRF and right MLF
• C Left midbrain involving the left third nerve nucleus
• D Left abducens nucleus alone

Explanation

This is a left one-and-a-half syndrome: a lesion combining the left PPRF (or abducens nucleus) causing ipsilateral conjugate gaze palsy, plus the adjacent left MLF causing ipsilateral adduction failure. Only contralateral abduction survives, hence 'one and a half'. The lesion must be in the left pontine tegmentum where these structures lie close together, typically from a paramedian pontine infarct. Option D is incomplete because an isolated abducens nucleus lesion would spare the MLF and preserve left adduction.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.