Review Question - QID 109255

A 6-week-old girl presents with her parents to the pediatrician for blood-streaked diapers. The patient is breastfed every 2-3 hours and voids 10-12 times daily. The patient previously had several soft stools per day that ranged in color from mustard yellow to dark green. Over the past 2 days, the patient’s stools have become looser and streaked with blood. The patient has also regurgitated several of her feedings in the last few days. Neither the vomit nor bloody stools have seemed to bother the patient. The patient’s weight continues to trend along the 50th percentile. The patient’s temperature is 98.4°F (36.9°C), blood pressure is 66/51 mmHg, pulse is 127/min, and respirations are 26/min. On physical exam, the patient appears well nourished and is cooing. Her abdomen is soft and non-tender. Physical exam reveals the finding seen in Figure A. Which of the following is the most likely etiology of this patient’s presentation?