A young anxious mother of a 10 month boy comes to you and requests a test for CF (cystic fibrosis). What is the most appropriate investigation?

The key is A. Sweat test. [The sweat test measures the concentration of chloride that is excreted in sweat. It is used to screen for cystic fibrosis (CF). Due to defective chloride channels, the concentration of chloride in sweat is elevated in individuals with cystic fibrosis].