A 10-month-old boy, previously healthy, was admitted with sudden irritation and crying. Beginning 6 hours ago, these symptoms were accompanied by intermittent colicky abdominal pain and vomiting. He looked pale and his legs were flexed against his abdomen. On physical examination, a right upper quadrant mass was found. Abdominal ultrasonography was requested.

Question:
Considering the clinical history and the presented images, what is the most likely diagnosis?

[Hospital Evangélico Cachoeira de Itapemerim (2018)] A 2-month-old boy was admitted in the emergency department with sudden and intense crying, accompanied by difficulty in evacuation and legs drawn up to the abdomen. Past medical history showed no constipation, only a runny nose and sneezing 2 days ago, with no fever or compromising the general state of health. Stool appeared normal at home, but during the pediatrician's evaluation he evacuated dark red and mucoid stools. There was also a painful palpable abdominal mass in the right upper quadrant.

Question:
What is the most likely diagnosis and the initial management?