Which of the following is the reagent used for the string test?

The string test is a diagnostic test primarily used to identify the presence of certain pathogens, particularly some Gram-negative bacilli, such as Vibrio cholerae. The reagent used for this test is 0.5% sodium deoxycholate, which acts to lyse the bacterial cell membrane, releasing cellular contents, including DNA, that can be identified by their ability to form a viscous "string" when the sample is mixed with the reagent and drawn between the fingers.

This property is significant because it not only indicates the presence of specific bacteria but also helps distinguish them from other organisms based on their cellular makeup and response to the reagent. In clinical microbiology, such tests provide rapid and reliable methods for initial screening and identification, which can be critical for timely treatment decisions. The other choices do not serve this specific purpose in the context of the string test.