Malrotation and volvulus — branching viva

Emergency department: a three-day-old term neonate, previously well and feeding normally, who has just vomited bright green bile. The baby is alert with a soft abdomen, normal pulses, no hernia, and a plain abdominal radiograph reported as unremarkable. The examiner asks: what is the governing rule and the immediate plan, which investigation confirms the diagnosis and what does it show, and what is the operation — then branches to the unstable child, the older child with recurrent bilious vomiting, and finally the incidental asymptomatic malrotation and whether it must always be operated on.