A 7-week-old boy is brought to the physician because of a 1-week history of bloody mucoid stools. He has not vomited during this time. He was commenced on cow milk-based formula at age 4 weeks. Growth parameters are within normal limits. Physical examination shows no other abnormalities. Which of the following is the next best step in management?
Abdominal ultrasound
Anal manometry
Stool pH test
Switch to hydrolyzed casein formula
Switch to soy-based formula
