A patient comes in eye OPD presents with diplopia,headache and blurring of vision. On examination while attempting right gaze, right eye abducts but left eye fails to adduct. Patient's right gaze is normal,convergence is also normal. The lesion is most probably in the following:-

Right cerebral coex is the command centre for right gaze origination. These signals runs through right paramedian reticular formation (PPRF) From PPRF signals reach VI cranial nerve nucleus of same side (Right here) which in turn supplies the lateral rectus causing abduction. Signals from Right PPRF also goes to III cranial nerve nucleus (LEFT) MLF (LEFT) leading to left eye adduction. Thus lesion in the LEFT MLF will result in failure of the adduction of the left eye, There is no effect on the movement of right eye. This is called Internuclear Ophthalmoplegia.