What is the diagnosis for a patient aged 6-18 months exhibiting symptoms such as vomiting, colicky abdominal pain, currant jelly stool, and a "sausage-shaped mass" in the right upper quadrant?

The patient's symptoms align closely with intussusception, particularly in the age group of 6-18 months. Intussusception occurs when a segment of the intestine telescopes into an adjacent segment, which can lead to obstruction and vascular compromise. Vomiting is a common presenting symptom due to the obstruction. The colicky abdominal pain is also characteristic, as it typically follows a pattern of intermittent severe pain.

The presence of "currant jelly stool" is a classic sign associated with intussusception, indicating blood mixed with mucus that occurs due to bowel ischemia. The "sausage-shaped mass" felt in the right upper quadrant represents the section of the bowel that has invaginated and is a key finding in a physical examination of a child suspected of having this condition.

In summary, the combination of age, vomiting, colicky pain, currant jelly stools, and a palpable abdominal mass strongly supports the diagnosis of intussusception, making it the correct answer.