This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory system, Subject name: Anatomy
Ina patient with clinical signs of asthma which of the following tests will confirm the diagnosis 
Select and explain. 
A. Increase in FEV1/FVC, B. >200 ml increase in FEV1 after Methacholine, C. Diurnal variation inPEF>20 %, D. Reduction of FEV1>20% after bronchodilators
C: Diurnal variation inPEF>20 %
Answer: C: Diurnal variation inPEF>20 % 
Explanation: Demonstrated outflow obstruction.,: Decreased FEV1 Decreased PEF Decreased FEV1/FVC Ref Harrison 19th edition pg 1675