mrcem2-2001

A concerned parent brings his 10-month-old son to the Emergency Department. He was previously healthy, but suddenly began vomiting this morning, and the father mentions that the last vomit was a vivid shade of green. The baby has been crying uncontrollably for periods of 15-20 minutes and then calming down in between. Upon examination, the child appears slightly pale, and there is no detectable bowel in the lower right quadrant.

What is the SINGLE most probable diagnosis?