Review Question - QID 108465

A 24-year-old female presents to the emergency department with a chief complaint of an inability to urinate. She states that this has been one of many symptoms she has experienced lately. At times she has had trouble speaking and has noticed changes in her vision however these episodes occurred over a month ago and have resolved since then. 2 days ago she experienced extreme pain in her face that was exacerbated with brushing her teeth and plucking out facial hairs. The patient has no relevant past medical history, however, the patient admits to being sexually abused by her boyfriend for the past year. Her current medications include ibuprofen for menstrual cramps. Her temperature is 98.6°F (37°C), blood pressure is 112/74 mmHg, pulse is 82/min, and respirations are 12/min. A physical exam reveals a pale and frightened young woman with no cardiopulmonary findings. A neurologic exam shows that gazing left results in only the ipsilateral eye gazing leftward. The patient's initial workup is started in the emergency department. Which of the following is the best initial test for this patient's chief complaint?