The following are multiple choice questions (with answers) about medical knowledge.   **Question:** 4 year old boy presented with recurrent chest infections. Sweat chloride test was done, showed values of 36 and 42. What is the next best investigation to confirm the diagnosis ?   (A) 72 hour fecal fat estimation   (B) CT chest   (C) Transepithelial nasal potential difference   (D) DNA analysis of delta F 508 mutation
['(C) Transepithelial nasal potential difference <> answer', 'The following are multiple choice questions (with answers) about medical knowledge. <> instruction', '(A) 72 hour fecal fat estimation \n (B) CT chest \n (C) Transepithelial nasal potential difference \n (D) DNA analysis of delta F 508 mutation <> possible answers', '  4 year old boy presented with recurrent chest infections. Sweat chloride test was done, showed values of 36 and 42. What is the next best investigation to confirm the diagnosis ? <> context@']