A 17-year-old adolescent female is 6 weeks postpartum. She presents to the emergency room with the complaints of increased jaundice, abdominal pain, nausea, vomiting, and fever. Her examination is remarkable for jaundice, pain of the right upper quadrant with guarding, and a clear chest. Chest radiographs appear normal. Which of the following tests is most likely to reveal the cause of this pain?