A 23-year-old African-American presents with acute-onset pain in the abdomen, back, and legs. On physical examination, his pulse is 115 bpm, respiratory rate is 20, blood pressure is 100/70 mmHg, and temperature is 101. There is scleral icterus, a s ystolic ejection murmur at the right upper sternal border, bilateral rhonchi, a right upper quadrant abdominal scar from a cholecystectomy, and a diffusely tender abdomen without rebound. A neurologic examination is normal. A peripheral blood smear is shown in Figure
.
A CBC with differential and platelets later shows the Hgb to be 6.4, white blood count is 2100 with 85% polymorphonuclear forms, platelet count is 100, and the reticulocyte countis 0.5.
Which of the following would be an appropriate next test?