What is the most likely diagnosis for an 18-month-old with episodes of severe abdominal pain and 'currant jelly' stool?

The presentation of severe abdominal pain accompanied by 'currant jelly' stool strongly suggests intussusception, which is a condition where one segment of the intestine telescopes into an adjacent segment, leading to obstruction and ischemia. In young children, such as an 18-month-old, intussusception is a common cause of acute abdominal pain and can present with this distinctive type of stool due to the mixing of blood and mucus.

The 'currant jelly' stool results from the stool being mixed with blood and mucus, indicating that the blood supply to the intestines is compromised, which can happen with intussusception. This classic sign, along with the episodic nature of the pain, points specifically to this diagnosis.

In contrast, hernias, appendicitis, and gastroenteritis do not typically present with 'currant jelly' stools. A hernia may cause pain and obstruction but usually lacks the specific stool characteristics seen in intussusception. Appendicitis can also cause severe abdominal pain, particularly in older children, but the stool would not present as 'currant jelly', and instead, would more likely be normal or have other patterns. Gastroenteritis usually presents with diarrhea, vomiting, and possibly fever, but