Review Question - QID 109266

A 2-year-old male presents to the pediatrician for abdominal pain. The patient’s parents report that he has been experiencing intermittent abdominal pain for 2 days. Each episode lasts several minutes, and the patient seems to be entirely well between the episodes. The pain seems to improve when the patient squats on the ground with his knees to his chest. The patient’s parents also endorse decreased appetite for 2 days and report that his last bowel movement was yesterday. The patient has had several episodes of painless bright red blood in his stools which the pediatrician attributed to a milk allergy. His parents were not concerned at the time because the patient did not seem to be in any pain. They deny any other recent upper respiratory or gastrointestinal symptoms. The patient’s past medical history is otherwise unremarkable. His temperature is 98.2°F (36.8°C), blood pressure is 71/53 mmHg, pulse is 129/min, and respirations are 18/min. The patient is happily playing in his mother’s lap. His abdomen is soft and non-distended, and he is diffusely tender to palpation over the entire right side. A 2x4 cm cylindrical mass can be palpated in the right upper quadrant. Which of the following is most likely to be found in this patient?