(Click the link to comment and to vote – voting not working through email, sorry!)

You are seeing a 3 year old child adopted from another country 6 months ago for abdominal pain. Since then, the child has had chronic nasal congestion and cough as well as failure to thrive which was expected to improve with improved nutrition provided by the adoptive family but has not. The child is afebrile and there are no ill contacts. On exam, there are scant wheezes and O2 sat is 95% on room air. CXR shows hyperinflation. Chart review shows prior visits for respiratory illness, and an episode of rectal prolapse. When asked about the presence of constipation, the parents describe frequent smelly oily stools.

What diagnostic test should be performed?