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: Microbiology
Reidel walker test measures efficacy of disinfection by using? 
Select and explain. 
A. Phenol coefficient, B. Glutaraldehyde coefficient, C. Ethanal coefficient, D. Formalin coefficient
A: Phenol coefficient
Answer: A: Phenol coefficient 
Explanation: Ans. is 'a' i.e., Phenol coefficient There is no reliable test available to determine the efficacy of a disinfectant. This is due to the number of parameters which influence disinfectant activity. Traditionally in such tests phenol is taken as the standard. Two impoant tests are :- i) Rideal-Walker test Suspensions containing equal numbers of typhoid bacilli are submitted to action of varying concentration of phenol and of the disinfectant to be tested. The dilution of test disinfectant which sterilises the suspension in given time, divided by the corresponding dilution of phenol, is stated as the phenol coefficient of disinfectant (phenol-l). In this test disinfectant react directly without any organic matter being present. ii) Chick-Main test In this, the disinfectant acts in the presence of organic matter (dried yeast or feces).