Review Question - QID 214911

A 29-year-old man presents to the emergency room with a 3-week history of fever, abdominal pain, and frequent episodes of small-volume bloody diarrhea. He denies any dizziness or chest pain. He refuses to disclose any information about his medical history. Review of the medical records shows he was discharged from the hospital 2 months ago after treatment with trimethoprim-sulfamethoxazole for Pneumocystis pneumonia. He reported at the time that he was supposed to be on medications to prevent that infection, but he had not been taking any of his medications. He has an active history of intravenous drug use and endorses smoking a pack of cigarettes a day. He has never traveled outside the country. His temperature is 102.2°F (39°C), blood pressure is 120/80 mmHg, pulse is 103/min, and respirations are 24/min. He has a normal S1 and S2 on cardiac auscultation. His lungs are clear bilaterally. There is vague abdominal tenderness in all quadrants. His CD4 cell count is 37/mm^3. Which of the following would be most helpful in establishing the diagnosis?