Twо dаys аfter undergоing uncоmplicаted total knee replacement, a 55-year-old man develops increasing anxiety, agitation, hand tremor, and nausea. He told the nurse he saw a bear in his room. His pain has been controlled with intravenous morphine. He has a history of advanced liver disease. He drinks 7 cans of beer daily. He is diaphoretic. His temperature is 37.6°C (99.7°F), pulse is 118/min, respirations are 18/min, and blood pressure is 146/92 mm Hg. Administration of which of the following drugs is the most appropriate next step in treatment?