A 65-year-old male presents with sudden onset dysphagia
and dysarthria, vomiting hiccup and vertigo. On examination he has a right
sided Hornerís syndrome, right-sided cerebellar ataxia, loss of pain and
temperature sensation on the right hand side of the face and loss of pain and
temperature sensation in the left upper and lower limbs.
In this patient the lesion is in the: