This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Miscellaneous, Subject name: Medicine
Which of the following investigation would be diagnostic of cystic fibrosis? 
Select and explain. 
A. Deficiency of the enzyme mucinase, B. High sweat chloride content, C. Alpha-1 aldolase deficiency, D. Increased copper excretion in urine
B: High sweat chloride content
Answer: B: High sweat chloride content 
Explanation: A sweat test measures the amount of chloride, a pa of salt, in Sweat. It is used to diagnose cystic fibrosis (CF). People with CF have a high level of chloride in their sweat. CF is a disease that causes mucus build-up in the lungs and other organs. It damages the lungs and makes it hard to breathe Ref Davidson 23rd edition pg 944