This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: AIIMS 2017, Subject name: Microbiology
Diagnosis of CI. Difficile infection is made by which of the following methods? 
Select and explain. 
A. Toxin gene detection by polymerase chain reaction (PCR), B. Culture, C. Enzyme - linked immunosorbent assay (ELISA), D. Nagler's reaction
A: Toxin gene detection by polymerase chain reaction (PCR)
Answer: A: Toxin gene detection by polymerase chain reaction (PCR) 
Explanation: Clostridium difficile is difficult to culture. The best method for its diagnosis is PCR for tox gene. Nagler's reaction is done for Cl. Perfringens. It is a neutralizaton reaction.