Lung cancer

A 75-year-old woman comes to the emergency department because of a severe headache and double vision for a day. She says she has recently also fallen several times because she feels unsteady when she is walking. She denies neck stiffness or pain. She has a history of lung cancer that recurred last year and was complicated by liver metastases. Examination shows that her voice sounds hoarse. She is afebrile, but drowsy. She is able to follow commands and is able to move all four of her extremities. Pupils are 4 mm bilaterally. Which of the following is the most likely diagnosis?