A 5 year old with recurrent chest pain, finger clubbing with offensive stool. Choose the single most likely investigation?

The key is B. Sweat test. [Recurrent chest pain from frequent lung infections including pneumonia or bronchitis."Clubbing" of the fingers is a classic features of Cystic Fibrosis, although not present in many patients. The digestive enzymes are not being produced, food is not adequately digested (malabsorption) and excess fat and protein is lost in the stools, making them bulky, oily, smelly and difficult to flush away].