This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pediatrics
Lactose intolerance in a suspected patient is diagnosed by giving an oral load and measuring concentrations of ? 
Select and explain. 
A. CO2, B. H2, C. One/two carbon compounds, D. N2
B: H2
Answer: B: H2 
Explanation: Hydrogen breath test is used for lactase deficiency and secondary lactose intolerance.