A 58-year-old man has sudden vertigo and a hoarse voice, and chokes when he drinks. The palate elevates to the right and there is no gag response when the left side of the pharynx is touched. Pinprick sensation is reduced over the left side of the face and over the right arm and leg. His tongue protrudes in the midline, his face moves symmetrically, both eyes abduct fully and power is normal in all four limbs.
Damage at which one of the following sites in the brainstem best explains this man's palatal weakness and sensory findings?