A 40-year-old woman is recovering from a tricyclic antidepressant overdose and is about to be transferred out of the ICU. She complains of sore throat, dysphagia, stiffness in back of the neck, and dyspnea. Even though she required endotracheal intubation and mechanical ventilation during treatment and the intubation had been difficult, she was successfully extubated 48 h before without any respiratory complaints. Temperature is 39°C, BP is 150/88 mm Hg, pulse rate is 98 beats/min, and respirations are 22 breaths/min. With the exception of bilateral coarse breath sounds and neck stiffness, the rest of the physical examination is unremarkable. Kernig and Brudzinski signs are absent. Which of the following studies should be done next?