This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: general microbiology, Subject name: Microbiology
Phenol test or Reidel walker test is done to determine? 
Select and explain. 
A. Hardness of water, B. Chlorine demand, C. Quality of disinfectant, D. Efficacy of a disinfectant
D: Efficacy of a disinfectant
Answer: D: Efficacy of a disinfectant 
Explanation: TESTING OF DISINFECTANTS There is no single reliable test available to detrmine the efficacy of a disinfectant. This is due to the number of parameters which influence disinfectant activityy. Traditionally in such tests phenol is taken as standard. In Rideal -Walker test, suspension containig equal number of typhoid bacilli are submitted to the action of varying concentration of phenol and of disinfectant to be tested . REF:Ananthanarayan & Panicker's Textbook of Microbiology 8th Edition pg no : 38