An 8-year-old child presents to the office with several episodes of diarrhea each day for 2 months. The stools are watery, without mucus or blood, and are not associated with crampy abdominal pain. Physical examination reveals a cachectic, malnourished child with a protuberant abdomen. The child's abdomen is tympanitic with a succussion splash. Which of the following is the most likely diagnosis?
a) Lactose intolerance
b) Giardiasis
c) Crohn disease
d) Short bowel syndrome