A 28-year old right-handed man with no prior medical history visits the opthalmologist for followup after refractive surgery for unilateral astigmatism the previous month. A normal left fundus is visualized, but an incidental finding is made of a small foul-smelling suppurative abscess in his right anterior hard palate. He is immediately sent to oral surgery and the abscess is drained; cultures are pending but results of a PCR-based assay are specific for P. aeruginosa. LP is obtained with normal opening pressure and clear appearance, but CSF cultures and chemistry are not yet back from the lab. A transesophageal echochardiogram is negative.

The nursing staff notes that the patient suddenly exhibits a significant degree of dysphagia during lunch. On afternoon rounds, the patient's speech is slurred and he has developed a lower right facial droop with significant right facial numbness. He recent onset of nausea, photophobia, tinnitus, nuchal rigidity, and a moderately severe holocranial headache that is worse when standing. His left pupil is dilated and hyporeactive, while his right pupil is reactive and normal. The remainder of the neurological exam is unremarkable; a CT is obtained and is negative for hemorrhage. J D Miles MD PhD orders you to administer a massive bolus of IV TPA, stat. Quickly, as you are drawing up the injection, what is the most likely diagnosis?