If anyone has an idea, there's still time. It's actually a characteristic presentation. I'll post the correct answer in another three days.
Correct answer: Coeliac disease. The characteristic buttock atrophy and distended abdomen are obvious (and less likely to be associated with kwashiorkor in a white person). In coeliac disease, IgA antibodies are formed in reaction to certain proteins in gluten. It eventually leads to villous atrophy, malabsorption, diarrhea and failure to thrive.