Which gram-negative diplococci can be presumptively identified from a positive oxidase test?

The correct choice indicates that all the listed organisms—Neisseria gonorrhoeae, Neisseria meningitidis, and Neisseria lactamica—can be identified using a positive oxidase test. The genus Neisseria, which contains these three species, is characterized as gram-negative and has a distinct morphology of diplococci (paired spherical bacteria).

A positive oxidase test is specifically significant in the identification of Neisseria species because they possess cytochrome c oxidase, an enzyme used in the electron transport chain. This enzyme catalyzes the oxidation of cytochrome c and reduces oxygen, leading to a color change in the oxidase reagent when the test is performed. When any of the three species—N. gonorrhoeae, N. meningitidis, or N. lactamica—are cultured and tested, they will yield a positive result, confirming their identity within the Neisseria genus.

In practice, this means that regardless of which specific Neisseria species is being tested, the positive oxidase test serves as a reliable method to presumptively identify any of these organisms as gram-negative diplococci that could potentially cause various infections. Thus, labeling all the options in this context