Internucler opthalmoplegia is caused by demyelenation of the medial longitudinal fasciculus carrying the stimulus to the oculomotor nucleus on one side to contract the medial rectus in unison with the opposite lateral rectus. Hence, the eye on the side of the lesion fails to look medially on lateral gaze. There is another H-test that covers this once you've solved the current case. Click the return link to see how this case differs from the above image.