A 12 month old male child suddenly draws up his legs and screams with pain. This is repeated periodically throughout the night interspersed with periods of quiet sleep. When seen after 12 hours the child looks pale, has just vomited andpassed thin blood-stained stool; there is a mass around umbilicus. What is the most likely diagnosis
Appendicitis Intussusception Gastro-enteritis Roundworm obstruction