Three policemen, with difficulty, drag an agitated and very combative young man into an emergency room. Once there, he is restrained because he reacts with rage and tries to hit anyone who approaches him. When it is finally safe to approach him, the resident on call notices that the patient has very prominent veical nystagmus. Sholy thereafter, the patient has a generalized seizure. Which of the following substances of abuse is the most likely to produce this presentation?