A 4 year old child presents with repeated chest infections. He has yellow discoloration of sclera and the mother gives a history of diarrhea as well. What is the single investigation most likely to lead to a diagnosis?

The key is A. Sweat chloride test. [Repeated chest infections, and jaundice suggests dx of cystic fibrosis. In the hepatobiliary system which is damaged in about 25 to 30% of the cystic fibrosis patients, malfunctioning secretion can cause liver disease and gall stones, leading to pain and jaundice. For cystic fibrosis sweat chloride test can be done].